I am running Excel 2007 and have been working on files and ONLY saving them to my USB memory stick.
On connecting the device to my PC (using an extension USB cord) I noticed a very brief minute blue like spark, and didn't release until I tried to access my files that the entire content was wiped.
Excel cannot open the file 'LostFile_XLSX_608879.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.
Well I think it looks fine to me, and I endeavored to use the Open & Repair when opening the file too, with no luck.