Why is cold water coming out of my water heater?
If your gas water heater is not working, first check to see if:

 - Is the water heater leaking? If so, turn off the water and gas to the heater, and contact a plumber to replace the heater.
 - Do you smell gas around your heater? If so, you likely have a gas leak--turn off the gas line to the heater and contact a plumber to replace the heater.
 
If neither of these are issues, check to see if the pilot light for you heater is lit. This is likely hidden behind a panel or door on the bottom of the front of the heater.  If the pilot light is not lit, follow the instructions to try re-lighting it.

If the pilot light does not light or continues to go out after heating the water, the thermal coupling on the water heater likely needs to be replaced. Contact a plumber to have this replaced.

Generally speaking, traditional tank-based, gas water heaters have a lifetime of 10-15 years. If the age water heater is within this range, you may want to prepare for replacing the water heater soon.