1991 yamaha 150 will not shift

Could be a few reasons, I once had a small bolt found the engine that had gotten lodged between the linkage and housing where the linkage exits the housing.

Check prop to make sure that the bearing casing ring did not loosen, Check prop to see if it turns if shifter is in neutral position.

Check shifter lever, is release button broke?

If this happened on the water, you may have damage inside the lower unit. if so, rebuilt time.