I agree with Chuck, first check the fuse(s) and flasher.
But if still a no go, check the igintion switch on the steering column.
I had the same problem on the boys 89 GT mustang, the ignition switch was loose in the socket, pushed the bulkhead back up in the switch and re-tightened the crimps on the switch and that fixed the problem.
I have worked at several Ford dealerships over the years and have seen this many times. It doesn't always affect the turn signals though.
The ignition switch was one of the first things we checked when a car came in having unexplained electrical problems, 9-10 it was a loose ignition switch.
Goodluck, Cp