Excel 2007 : Unable to share a previously shared workbook

I have a very simple workbook that was sharing perfectly with 2 co-workers. We had a quick power surge and now it won't share. I have tried unsharing it and sharing it again. I have tried saving it as a different name, in different places. Now I have multiple versions and it still won't share. I am a self-taught Excel user. Have I missed something? How can I get rid of all the redundant workbooks? Thanks for any help. PS: I run 2007, not 2003 as noted in profile.