Welcome to Just Answer, I will do my best to help you solve your problem
Is this an electric water heater?
It's under the top access panel. Turn off the power then remove the panel and the insulation behind it. Then press the red reset button. If it clicks you have reset it
the button is at the top of the thermostat
ok, if this does not work what would be my next step?
If the button doesn't click then you could have a faulty upper thermostat or upper element if you have NO hot water. If the water is only hot for a short time then the lower thermostat and element are suspect
the only way to verify which is the problem is with a multi-meter
thanks I will try this. I'm thinking one of the elements is at fault, though. Heater is almost 15 years old
Changing the thermostats is easy. If the reset works you should change both upper and lower stats since it will happen again. Changing an element involves draining the tank
if you do change an element be sure to remember to fill the tank fully with water (open a hot water faucet to be sure the air is out) before you turn the power back on