I don't have hot water suddenly. Please help me.

Check that you are receiving 240Volt across top 2 screws on upper thermostat.
Copy following link for troubleshoot resources:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Open following links for water heater troubleshoot:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

This sounds like the dip tube has dropped off ... If that has happened, there is no repair. Seek the guidance of your plumber n this please.

1) Call Rheem. I don't know the number outside the US.
1-800-432-8373
2) Contact local plumbing supply and ask who sells the Rheem unit. Then call that dealer and inquire about rating.

You need to first determine if the burners light or not.

As long as the unit wasn't run with a dry tank the unit should not have been affected by continuous water running, unless it got hot enough to damage the thermocoupler which is an easy replacement. That is the part that your pilot/ignitor warms so that the gas valve will open the gas line to the burners.

You have 2 problems with several possible causes:
You have 10-year-old electric water heater that was reinstalled after falling down, and doesn't heat the water.
And you have no hot or cold water pressure, and no pipes are leaking.

1) First problem is lack of water pressure throughout house.
Turn on outdoor spigot nearest to water meter.
This spigot will tell you if problem is inside the house or between the house and meter.
If spigot runs full force, then you probably have clogged pipe where water heater connects to main cold line, or the main shut off valve got clogged with sediment that broke off inside the pipe when you were working on water heater.
Open and close shut off valve to see if that helps. Then disassemble pipes and track back the problem. Put pressure gauge on water heater drain valve to watch pressure drop when turning on indoor faucet.
Read following information:
http://waterheatertimer.org/Low-hot-water-pressure.html

Water heater doesn't work:
Test across wires at top of heater to see if you are getting correct voltage:
If tank is receiving correct voltage, then no hot water puts suspicion on upper element or upper thermostat, or both.
Open following link for electric water heater troubleshoot steps:
http://waterheatertimer.org/How-to-troubleshoot-electric-water-heater.html

Consult your owners manual.
http://www.bosch.com.au/content/language1/downloads/70R_UserManual1023.pdf
Heres a new one if you don't have or lost yours. You can't, you got bad advice. tankless system don't have a running pilot light. They operate by detecting water flow and then turning on. Many also state on the lighting instructions to NEVER try and manually light the pilot. Try Bosch tech support. Lowes sells Bosch heaters and they might be able to help you get the support number faster. Hope this helps but please don't go sticking a flame inside there.

Without model number I can't tell you specifically where they are but, it is all controlled by a thermostat. If it's manual light, the gas control has the thermostat built in as part of it, and if water is getting too hot, even with temperature turned down, replace gas control. There is a temperature lever on top that moves from side to side to adjust temperature. If it's electronic ignition, the thermostat and ECO should be visible below pressure relief valve when you open the cover. On the electronic models, the thermostat disc must be up against the tank properly to do it's job. If it's tight against the tank, and water heater still overheating, replace thermostat. One other thing to check would be incoming water pressure. If on city water connection and pressure is too high, it will also cause pressure relief valve to open prematurely, but if water is obviously too hot, that probably will not be the cause. Please remember, if work is required on the propane system, it should only be performed by a certified technician. Just this week alone, I've repaired 3 RV's that could have caught fire or worse, because they tried to do it themselves. Don't endanger yourself, your loved ones, or your friends and neighbors, to save a 1/2hr, labor charge.

If you had to shut the entire house off to install, your main valve may have broken off. Common problem with cheap Home Depot gate valves. Check valve and see if it turns endlessly. If so, that's the problem. To get water back on you'll need to shut off at the meter. The stem assembly screws into the valve body. Carefully take a wrench and unscrew stem assembly from body. Once removed you will need a pair of needle nose pliers to remove gate assembly which broke off. Pull out gate, make take a little effort if water is hard in your area, may have to tap a few times with small hammer on bottom of valve to free it up. After removing gate, reassemble whats left of the stem assembly. Be careful with gasket, grease on threads helps. Tighten vice and snug. Note: Whenever you are working on piping, plumbing systems, always be aware of how you tighten and loosen things up. Back up what your removing with another wrench or channel locks. You don't want to snap something off and create more problems!

Now go back inside and open hot and cold water in tub. Make sure valve is on over water heater. DON'T PLUG IN OR TURN ON ELECTRIC WATER HEATER UNTIL YOU KNOW IT'S FULL.
Turn water back on at meter. Go inside and see if water is coming out.

The manuals don't mention a battery.
Open following link for Bosch contact information and troubleshoot resources.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Bosch gas 10p.
Word search of manual does not show battery.
Save the manual for future problems.
http://waterheatertimer.org/pdf/Bosch_10P-installation.pdf
http://waterheatertimer.org/pdf/Bosch_10P-water-wizard.pdf

Bosch electric under-counter P10-P15 resources:
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/AristonModels/P10P15Models/Troubleshoot/tabid/653/Default.aspx

Bosch P10 manual does not show battery during word search
http://waterheatertimer.org/pdf/Bosch-P10_P15_English.pdf