Hello. First remove the upper thermostat cover plate and check the high limit switch button. If it has tripped, press it to reset it.
Check for power at the input terminals of the upper thermostat. (usually marked L2 and L2) If no power is present then check your circuit breakers or wiring to the water heater.
If there is no hot water then the upper thermostat should be applying power to the upper heating element. Check for voltage at the upper heating element. If there is power at the heating element but the element is not getting hot then the heating element needs to be replaced.
If there is no power at the upper heating element then the upper thermostat needs to be replaced. Joe