There is a cruise control switch up under the foot well. The clutch pedal presses against it when not in use. Cruise control will automatically deactivate if the clutch pedal is depressed.
I would check the position of the switch first - make sure the pedal is in contact with it and depressing the switch. Second, I might try to take apart the switch and clean the contacts inside - people have done this with another switch that is activated when the clutch pedal is fully depressed. Third, I would try replacing the switch. I don't think they're terribly expensive, so it's a good start for troubleshooting
