Hmm... has it by any chance lost what its meant to open it with?
Does it have the Excel icon on the files? I'm guessing if this has happened, it won't have.
Right click a file, and go to Open With.
Select Choose Program and choose Excel from the list if its there. If not, do a search for the Excel EXE file in one of your drives.
Make sure it is selected to always open this kind of file with the program and click Apply and Okay.
That should do it