Why am i getting this error msg in excel for office 2007?

i'm getting an error msg when i try to open an excel file that states: "excel cannot open the file 'summary.xlsx' because the file format or extension is not valid. verify that the file has not been corrupted and that the file extension matches the format of the file." the odd thing is this is a file i use weekly, & it was opening fine until today. any idea what might have caused that & how i can correct it? i have files saved on a flash drive & all files that i saved under the xlsx (office 07) format has the same error msg while all other files that i have saved under xls (office 97-03) opens up fine. appreciate any istance you can provide.