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?

The ".xlsx" file extension name was introduced with Microsoft Excel 2010, so your old version won't recognize it. You might try renaming the file with ".xls" file extension name, so that your Excel 2007 recognizes it, but it may have problems opening the file properly. Alternatively, whoever created this file should be able to provie yuo with a version using the ".xls" extension

Umm the xlsx extension was introduced with 2007, not 2010. However, if the files were saved in 2010 with 2010 specific features, that could cause the error.

However, you say these are files you use regularly, so that shouldn't be the problem. Its probable that something hit your hard drive causing the corruption.

Step 1. Reboot then try again.
Step 2. Run a Disk cleanup to do a Scanfix.
Step 3. Recover from backup (You do have backups don't you?)