Steve,
I am assuming that you lost the high speed on only one fan. If that is true that one of your fans only runs on low speed and does not run at all on high speed, the problem is a bad switch or a bad contact/broken wire between the switch and the fan motor. It will be very easy to diagnose the problem, just take a good volt/ohm meter and check for voltage at the switch and continuity in the circuit between the switch and motor.
Gerald