Save function not working...but I don't want to close down project to fix it

Hi InDesign experts,

I have been working in a large file (typsetting a book) for a couple of days without closing InDesign and then sometime while I was working today I went to undo and... it won't let me. I went to save and... it won't let me. 

When others have had this problem I've read their suggestions to close InDesign and reset the preferences or update the app. The problem is, I don't know when it last saved and I fear I'm going to lose a lot of work if I close it down now. 

Any solutions to bringing back the save functionality--even just temporarily so I can save and close it down-- without closing my file and losing all my work?

Thanks in advance!