I'm using Excel 2000 on a Windows 10 computer.

One of the Excel files that I've been working on for several years all of a sudden will not sort on any of its worksheets. Yesterday it worked fine. All the other files sort correctly. Today that file's Sort function is greyed out. I compared the options from a good file to this file and everything is identical. I restarted Excel but it did not fix the problem. I restarted the computer and that did not fix the problem.

What else can I check to see what the problem might be?