I finished making changes on a document to discover not only will it not save, I can't save as either. I created a new document and tried to paste some of the critical parts from the other document into it before I force-quit InDesign, and the minute I paste something into the new document I lose the options to Save or Save As.
If you haven't alread closed ID, don't, at least not yet. Try exporting to .idml, then force-quit so you have recovery data (if you shut down normally, the recovery data will be lost and you won't have any of your new changes anywhere).
Reboot, then restart ID, and with luck the recovery file will open and be savable to a new name, as will the .idml file.
That worked! Thank you!
Count your blessings.
If you don't already have a robust backup system in place, this should be a wake-up call for the need to do it.