I had an inboard Volvo on a boat that this happened to. My mechanic friend told me to take out the spark plugs put a little marvel mystery oil in and let it soak for a couple of days. After two days I tried to crank and it cranked right up. I hope this helps.

Lots of penetrating oil and patience. or fill it up with diesel. either way it pays off to work it back and forth, even a little bit to start with.