<p>I have renamed some excel files on my web server and after renaming the users cannot download those files. What could be the problem. They are all excel files.</p>

Some possibilities I can think of:
Make sure the extension on the files is still .xls (that's how it associates the mime type)
Check the file protections on the files in case it, or the ownership got changed when you renamed them
Make sure Excel didn't open them on the webserver (if a windows web server) as it will keep them locked until the EXCEL.EXE process is killed.
Might be one of those ...