Found a work around my exporting as a imdl. Would still be interested to know what caused the issue.
In such situation , its better to export the file in IDML to atleast save the changes made in the file,
So once you have exported all the file , force quit insdesign and then Replace your Indesign Preferences
An unable to save message usually indicates you either don't have permission at the location, or the file is marked as read-only. You didn't put it on a disk, did you?
Beyond that, file or prefs corruption would be high probabilities, so the .idml route, and replacing the prefs are both good suggestions. I would NOT force quit by choice unless you are unable to export the file. The force quit will give you a file in the recovery folder that may or may not be corrupt, but in cases where you cannot save and cannot export it's basically your best hope, and after the force quit immediately replace the prefs (see Replace Your Preferences) and if the file opens on auto-recovery immediately try to export it again.