You need to check the lower shift cable. When they get corroded, you lose the ability to put the outdrive into reverse. The easiest way is to pull the outdrive and turn the shift yoke to the reverse position and see if it engages. If so, replace the lower shift cable and inspect the shift bellows for holes or cracks and replace it if you find any.
I recommend a manual for replacing the lower shift cable. It is not a staight forward procedure.
, just joking, that is the worst case though for real.
