Belts normally turn over either because they are worn out, or because the pulleys are misaligned. Since you know the belt is worn out, that is likely to be the explanation in this case. When a belt turns over, its widest part is innermost on the pulley, instead of outermost. That means it rides way up high on the pulley, since only the top of the pulley groove is wide enough for the belt to fit. If it isn't clear why that is so, draw a diagram of a V inside another V, and note that they "nest" into each other. Then turn the inner V around so its wide part is trying to enter other V: it can't fit in, it just rides on the outside of the other V. Now, think about the V belt: its effective length is not its outermost length, it is about a third of the way inward toward its innermost length. It would have to be somewhat longer than its effective length, to fit around the outside of the V pulleys. Since it doesn't get any longer when it turns inside out, it is suddenly too short to fit around the pulleys, so the clutch pulley doesn't retract far enough to relax the belt. You could probably get it running in the short term just by adjusting the clutch linkage to make it retract the clutch pulley further, but what you need to do to fix the problem, is replace the belt.
If you post pictures of the pulleys, belt, and belt routing, we can probably work out how to remove the old belt and fit a new one. If you and I can't, we're fairly sure to have other members who can.
