Excel help, file corrupted

I'm new here and I'm not sure if I'm posting in the right place....anyway, here's my problem. I'm trying to open some excel files with excel 2010. I'm getting a message that says "excel cannot open the file "filename.xlsx" because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file.

This just all the sudden happened to some files on my flash drive. its not all of them but its alot of them.

Also a couple of my folders on the flash drive kind of disappeared. but not really...I have a folder named "reports" and like I can see it but its not a folder I can't open it. When I click on it it brings up a bunch of programs to choose from but none of the programs work because its supposed to be a folder with excel files in it. So I'm not sure what happened but any help i can get would be great. thanks!