My Mountain bike was working perfectly last week. I went for a ride at the weekend and the rear derailleur has developed a problem. There is no spring in the derailleur. The lowest jockey wheel has moved towards the crank and the chain is sagging, previously the lowest jockey was properly inline with the upper jockey wheel. The cable is still in the same position as it was when the derailleur was working correctly. Feedback appreciated as I need to go for a ride.