Try the following steps to check the issue:
Check the file permissions:
You can do this by clicking once on the file to highlight it, then selecting "Get Info" from the "File" menu at the top of the screen in Finder, to see if you have "Read & Write" permissions to this/these files.
Check whether your drive have been corrupted, and a permissions repair should fix this if it is the case.
Check to see if it's just a permissions problem by trying to open the file in your text editor. You shouldn't see a nice, ordered spreadsheet when you do this (what you should see is random garbage on the screen), but you shouldn't get an error message about not being able to open the file or files.
Try to copy the file out onto your Desktop and try to open it from there (a copy of the file and not the original), check to see whether it will give the same error message.
If that does not work try to repair your Office installation