Try this:

1. Open Excel 2010.
2. Click on File > Options.
3. Select Trust Center > Trust center settings.
4. Select Protected view.
5. Uncheck all the options under Protected View > OK.
6. Restart Excel 2010 and try to open Excel documents.
7. Once you have been able to open the document, save it with Office new format (i.e. xlsx)
8. Go to File > Options -> Trust Center -> Trust Center settings -> Protected View
9. Check all the options again under Protected View -> OK
10. Restart Excel 2010
The above will set the security options normal again. Although the process of setting back the security is a burden, it is necessary because if not, then you could forget (as in my case =) ) and could open any file without security...
Thanks!

If doesn't help look for the answer here: https://social.technet.microsoft.com...me?forum=excel