There's a chance the shifter has disengaged from the selector forks. The forks can be manipulated back into position with the shifter lever removed (it is a fiddly job but you will physically feel the tension on the selector linkage when you have got it in the right location) If the shifter bush is missing you will need to replace it (it's a nylon cup that clips onto the ball socket on the end of the lever assembly - without it you will get a lot of shifter slack and it will more than likely try to pop out of the main selector linkage again)