Hi - sorry about the trouble! Sounds like your save got corrupted somehow, unfortunately. Could you post your starsector.log file - or the last couple hundred lines of it, after you try and fail to load the game? That might give some idea about what went wrong.
As far as getting it back, there's a backup save in the save folder. The save folder itself is under saves/save_<pilot name>_<a bunch of numbers>. Within that, you should see 4 files:
campaign.xml
descriptor.xml
campaign.xml.bak
descriptor.xml.bak
What you want to do is move the .xml files somewhere else (they're the current, not-loading save) and then remove the .bak extension from the other files (they're your previous save). So you'd still lose some progress, but hopefully your previous save will load.
