Excel connot open file (filename) xlsx

I have several excel files stored on my system. I used these files from the same computer daily. Yesterday, when i tried to open several files in Excel 2007 then i found following error on my co,puter screen: "Excel connot open 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" How fix it?

Try changing the extension to .xls