Hi Billie,

You didn't mention the model number, but they are all pretty basic. The flame sensor and thermo pile which are thin flame sensors, get charred and need to be cleaned. They should be cleaned with a soft cloth or even a dollar bill will work (a worn one)

When the heater shuts down it usually means it is not sensing the flame and this will cause it to shut off automatically. I have included a link to get the owners manual below.

Good luck and be sure to turn off the gas and power to the unit before working on it.
Click Here to Get Owners Manual

Thanks,
Paul