Did you check the water heater to make sure is is not leaking or that a pipe busted someplace or a valve is closed?

First of all - did you check the obvious, like - if electric - has a breaker tripped or fuse blown? If fuel-fired - do you have spark/pilot?

Next, did you pay your utility bills? and/or re-fill your gas or oil tank?

Is the temperature set on your water heater (or did it get inadvertantly turned down to standby or vacation mode? or off?).

Do you have a direct vent fuel firing water heater? any obstructions outside where the vent terminates?

You have to provide more information. You might check your owner's manual and the labels on the water heater itself for some troubleshooting tips.

You say NO WATER COMES OUT AT ALL when you turn on the hot water.

If you mean you open a hot water valve and no water comes out of the tap - perhaps your water supply is turned OFF. Another thought - your COLD WATER INLET VALVE to your water heater is closed. In that case (obstructed or closed valve) your WATER HEATER SHOULD BE COMPLETELY OFF AND TAGGED OUT, for to fire it/have it on - can create a DANGEROUS SITUATION.

Does the COLD WATER RUN FREELY?