Port engine does not go into forward gear, reverse still works

[Captain Sly]

I have an '87 Weekender 300 with twin merCruiser 5.7L fresh water cooled straight inboards. This morning I was practicing docking (new boat owner!) and the port engine stopped shifting into forward gear. It still shifts into reverse just fine.

Mechanic is coming out tomorrow, but wanted to ask for some advice or suggestions about what could be the culprit and what I should definitely have the mechanic check out.

Thanks!