My water heater wont heat up - Rheem MR105245 Marathon Electric Water Heater 105 Gal

You posted under Marathon electric heater.
If you have tankless or tank-type gas heater, information below does not apply.

Absence of hot water say upper element is not working.
1) This can be caused by tripped reset button located on upper thermostat. Open upper cover and push in reset button. Put ear against tank to hear bubbly fizzing sound. Put insulation and cover back over thermostat. Continue reading information below because reset button trips when thermostats are bad or element is shorted.
2) Upper thermostat is bad. Replace upper thermostat. I usually replace both upper and lower thermostats.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html
3) Electricity going to water heater from breaker box is interrupted.
http://waterheatertimer.org/Test-electricity-to-water-heater.html
4) Upper element is bad. Test element. Replace element if needed.
http://waterheatertimer.org/How-to-test-water-heater-element.html
http://waterheatertimer.org/How-to-replace-water-heater-element.html

5) Add a comment for more free help.

6) Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on heater or any do-it-yourself project.
Fixya is always less expensive than a service call.