I'm assuming/hoping you have found the solution for this but if you haven't I hope it worked out ok for you. This does occasionally happen and the first time it happened to me I freaked out! However, there is a pretty decent work around so stay calm and hang in there - you'll get through it.
Here's what to do:
1. Go to FILE > EXPORT
2. Choose FORMAT > IDML
That will export a working and current version of your project.
BEFORE YOU QUIT - open the file that you just exported and test to make sure everything is ok.
If all is well, now save that as a proper InDesign file and then you should be good to go. You can quit and close the other file and should now have a working version that can be saved.