How can I fix this problem with my hot water heater?

Sounds like the heating element of thermostat is bad. Need to call a plumber.

Turn the temperature up a bit and see if that solves the problem.

You don't mention if this is electric heater or radiator style boiler. If electric, you do realize there are sometimes two heating elements (one at the top and one at the bottom). It could be that the bottom element is not working and is quite easy to replace. I would verify with an electrician to see what you need.

water heaters need to be drained and flushed periodicly also,to flush out dirt and sludge buildup.It's amazing the amount of crud I've seen come out of water heaters.Always remember to turn power off before working on any electric device.Once drained and blackflushed until completely clean,you can fill the tank with water then open the panels for access to your heating elements(assuming you have an electric unit).Fill tank complete with water so you don't damage heat elements,then using multimeter,read out the unit to see if both top and bottom elements are working properly.Now you see why it is important to contact a plumber/electrician?