Did this happen quickly, after one stop or did you notice the clutch slipping increasingly before? We need to know this before offering some suggestions.
I doubt the clutch cable is the problem. Usually if the cable slips you won't be able to disengage the clutch completely or at all. Only if the cable is too tight or binding somewhere would it cause your problem.
We might be looking at a broken or missing shaft key if the problem happened immediately.
Tom