Was there any plumbing work done at your house recently ?

OK! It goes from hot to cold after a short time? It sounds like you may have a dip tube that's broke.
The dip tube, (see image) is a white plastic tube that sets right below the cold water inlet and directs the cold water to the bottom of the tank to be heated and raise to the top. If it's broken it will direct cold water into the hot water at the top and temper it down to cold in a short while.
If you have the tools and soldering skills you may replace it yourself or it shouldn't take a plumber over a hour to replace it. Be sure to tell him your heater size and to bring along a new dip tube. Good luck, Tom