Moved from the Creative Cloud to the InDesign forum. They will be able to help you here.
No, but seriously, a bit more information from you would help us figure it out. It could be that you dragged a bunch of read-only files off of a CD (look at the Properties / File Info in your OS to check), or that you opened an InDesign Template that will let you use Save As but not Save (is Save greyed out in the FIle menu? Is Save As greyed out?) or it could be any one of a number of things. Some of those things are bugs; some are user error. Tell us about it and we'll see if we can't help you figure it out.
If you're having an emergency try exporting IDML (File -> Export and then pull down the "Save as Type" dialog and choose "InDesign Markup (IDML)." That way you can at least save your work if your file is damaged, or if it takes you a long time to figure out what is going on.
Have you downloaded the update to InDesign CC 9.1, released August 22?
What platform are you on?