Hello.
I am using MS Office 2007 on my PC and I have several excel files which I always use at work. Naturally I used these files daily until recently. Yesterday I have faced an error while I tried to open my working files on Excel: Excel cannot open the file .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 can I fix this error?