I have a 7 yr. old Trane furnace. (x80)Last night the furnace was running and blowing cold air.I turned it off @ the thermostat and waited a few minutes.I turn the thermostat up to 74,when the furnace kicked on it only blew cold air.I watched to see if the flames turn on but they did'nt ,Not even for a minute.

Did you see a orange glow after you shut it off and turn it back on you should see a orange glow and then after a minute to 90 seconds you should hear a click that's the firing valve

If you don't it could be your hot surface ignitor

The furnace should have a power indicator light. If it isn't on, you may have blown the control voltage fuse. If it is on, it should be blinking a trouble code if the furnace isn't working. To find the fuse or know how to respond to the trouble code, see the sticky at https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html