What is the EXACT message?
The EXACT message is:
"Cannot open Expansion.indd".
What operating system? What version of InDesign?
No more to the message?
Try a reboot.
Check for a .idlk file in the same folder and delete it.
Try Choosing the Open as a Copy option in the Open Dialog.
This problem has been driving me crazy!! I lost 20 hours work so far. The 'open copy' option worked!! Yay! Thank you P Spier!! The weird thing is I even tried opening older versions from dropbox and it still wouldn't work. This has worked though, thank you again