Sorry I didn't get back to you sooner. Been kind of hectic around here.
So take a look at what's happening when you pull the "START" lever. The spring arm on the clutch should move inwards, towards the engine. If not, you probably have too much slack in the cable. There should be very little space between the arm and the center point of the clutch hub.
If the arm is contacting the hub and moving in, but the clutch discs are not squeezing together, there's probably crud and corrosion that's got it frozen. You'll have to disassemble it and clean everything up. (This is actually a good idea even if it was working.)
If the plates squeeze together but the clutch doesn't turn, maybe the belt is loose like Slew Foot said.
Can you turn the engine over by turning the magneto on the right side?