The thermostat has a red light blinking and the fan has not cycled off. It also appears not to be blowing cool air. It worked fine this afternoon. What may be the problem?

Kill the power in breaker box to furnace and a/c for just a minute and turn them back on, see if it resets

I had one that flashed red when it was time to clean the filter. Impossible to tell what's going on with minimal information.

You might try looking at the instructions (owner's manual) online if you can't find the one that came with the unit.