Breaker could be bad.
Let's do a quick check and save you money.
Open top cover on water heater and push in red reset button that might be tripped.
If reset is tripped, water heater should turn ON.
If reset will not reset, then replace upper thermostat:http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html
If reset is not tripped out, then it is OK, so test two top wires on upper thermostat.
Use cheap multimeter from home center, and set dial to 240-250-277 VAC.
Test across top two screws on upper thermostat to see if you have 240Volts.
To do full troubleshoot on ordinary tank electric water heater takes 30 minutes and multimeterhttp://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html
If water heater is not receiving 240V, then the breaker is suspect, or a wire between breaker and water heater is loose.
See basic water heater circuithttp://waterheatertimer.org/240-v-water-heater-circuit.html
Now let's look at circuit breaker:
Mark suspect 240V breaker.
Identify all your breakers, and also mark another 240 Volt breaker that is same amperage as water heater 240V breaker.
Turn off main breaker.
Take front panel off breaker box.
The wires and lugs on main breaker are HOT, so stay away from that area.http://waterheatertimer.org/See-inside-main-breaker-box.html
Suspect breaker has 2 wires coming from breaker.
See if one or both wires are loose, and then tighten.
If wires are tight, then move these wires to the other 240V breaker that is rated for same amps as water heater.
Now turn on main breaker.
Put ear against water heater tank to hear bubbly fizzing sound.
This will identify if circuit breaker is bad, or if water heater needs a full troubleshoot.
It is not a good idea to run two 240V appliances on same 240V circuit breaker, so you may have to decide one or the other appliance until you can replace breaker.
Instructions to replace circuit breaker:http://waterheatertimer.org/How-to-replace-circuit-breaker.html
Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, fixya expert speaks with you over phone while you work on water heater circuit.