There has been a problem with some water heater diptubes breaking up. You would find white plastic chips in your showerhead and other faucet screens. If so then it may be under warranty and should be replaced by a plumber. If that was a problem, besides the white chips, you will eventually start losing the amount of hot water available as the diptube breaks off.
You could also have a plug at the faucet itself. Does the tub faucet work and the lav faucet?