Oh Man, don't reinstall your game - yet. If you can do a little Text editing with
Notepad on your Saved Game files ( or a copy of them just to be safe), you probably can save your Career.
It's a common issue, read through the posts & topics below and it may be an easy fix.
As
Raptor Pilot says, "One thing that's very important to note is, first save your game in port.
Then apply this fix.
Then start up and load your saved game"
At least we can fix it now. Usually happens if you change the gun from fore to aft or vice versa from where it was when first installed. Also happens sometimes when changing submarine classes.
Combine some of the above information, some talk about crew, some talk about guns & dates, but it is usually an easy fix on the Saved Game files.
Good luck and Happy Hunting!