I have a ES640 Series and the water is not longer hot. Could it be the thermostat? Can I replace myself?

It doesn't sound like a thermostat.
There are several things that would cause this problem,
1) A faulty did tube **OR**
2) A bad lower element
This will be a job for a pro. The element will have to be checked and the tank drained to replace it and to even check the dip tube the water shut off and the cold water inlet opened up. Good luck, Tom

Well yes and yes but Tom bring forward many other possible issues. You can replace a thermostat but at this point you have not determined that is the proble, you could be throughing money and time down the drain. We can do several "tests" if you want to narrow this down, do you want to do this?

Bump