Either it isn't shifting into gear or the clutch is toast.
Take the cover off the clutch. Shift into 1st gear, then rotate the rear wheel. Watch the clutch carefully.
Normally, you would not be able to rotate the rear wheel in gear without turning the engine over, which would be fairly difficult to do by hand in first gear.
If the rear wheel turns but you don't see any movement in the clutch at all then the transmission is not in a good state.
If one part of the clutch turns but not the other then the clutch is toast.
I recommend going to an online OEM parts dealer and examining the parts diagram to find out where that spring came from. There is a very good possibility that it is related to the problem.
Rod