Yup, that's why I joined the forum. I had the exact same problem.
Im sorry to say this, but you fallen victim to the Save Game Bug. It mostly occures when you save near a ship, land, or submerged. Altough this isin't a rule, it's safer to save while on the surface, away from land, and no ships in sight.
And before you ask, that save is propably gone for good. But try loading an earlier one, maybe you'll get lucky and manage to restore at least some progress.
Again, sorry to hear that. I lost a HMS Rodney (30.000 tones) due to this bug.
