Last month, during a 2 day cold spell, I turned the heater on and it worked fine. Since then I have needed the AC for only 2 days. Today, I turned the heater back on "heat" and it blew but never got warm. It is a gas heater/ac.
What is wrong? I don't see any "pilot light" or blue flame but then I'm not sure where to look.

Do you know how old the furnace is? Newer ones are electronic ignition and have no standing pilot.

The first step is to turn it back to A/C. If the blower continues, then you may have lost your control voltage. Go to the sticky at http://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html It explains replacing the control voltage fuse. If that doesn't fix the problem, read through the section on sequence of operations. If you need more help, post back with the details on what you have found.

Thank you to all who tried to help.
After sitting up in the cold attic for 1/2hr I found a wire that did not go to anything. I then found a place that it looked like it belonged to. So I attached it. Yeah!!!! It works now.