Try saving it as an IDML file, renaming it and trying to open in again. If that doesn't work you'll probably have to go back to your last incremental Saved version.
I can't save it as IDML becuase I can't open it...
How do I go back to past version?
Assuming you saved incremental versions, you open it in the usual way: File > Open.
If not you'll have to start again, unless that is someone, perhaps from Adobe, has the specialised tools to try and recover it.
It's a lesson learnt (and we've all been there!) regularly make incremental copies of your documents including copies to external devices and places, so if your computer breaks, is stolen or damaged you can still access your work.
If you have no back up you’re probably out of luck. You can try Markzware: http://markzware.com/file-recovery/recover-indesign-data-corrupt-indesign-dtp-file-recover y-service/
Also, give this a read to avoid having this happen in the future: http://boblevine.us/why-i-always-save-files-in-dropbox-and-why-you-should-too/
absolutely agree w/Bob about working from a Dropbox folder if possible. It's a lifesaver.
But I'd still do a TimeMachine backup (if you're on a Mac, which also backs up Dropbox local folder.