Can't edit macro, VBA project is unviewable

Running Excel 2010
I only edit this macro once a year so this is the first time I noticed that I can't edit it. When I click the project in VBA I get: Project locked/project is unviewable.
It's not a shared workbook.
I create a new file every month and looking back I see it stopped working in the April 2013 file. (don't know if that is relevant, just throwing it out there!)
I can't copy it in here of course because I can't view it. I can copy in the one from the March file, but I don't know if there are rules about that... I am a first time poster so if you want to help and need to see the code please let me know how to paste it in.
Any ideas???
Thanks in advance,
Julie

Anything is possible!
I checked permissions and "Anyone can open, copy, and change any part of this workbook"
Anyplace else I should check for password protection?
if not, any other ideas?

Nobody knows anything about macros around here except for me. Of course it could happen by accident, how is that done?
Rolling back would be a huge headache as there are 16 tabs all linked up to each other. Last resort.

STEVE YOU ARE A GENIUS!!!
Sharing and then unsharing worked. You have outsmarted MS - you should go work for them!
(wait, maybe you do... )
Thank you!