OK, clearing cookies and temp. internet files will delete your browsing history and also any passwords you have set to automatically log you in. So you will need to log into this site again manually.
Make sure you only have one window of Internet Explorer (IE) open. Click Tools, and then Internet Options. Then under Browsing History select Delete. You should have a menu of options, you can delete all if you like, or you can delete one by one and just exclude Passwords (doing this may save you having to enter passwords for all your sites). Deleting Temp. files could take a very long time if you haven't done it recently.
Once all operations are finished it's important to close IE entirely - all open windows. Wait a few seconds and then open again, log back into the site and see if that fixes your problem
As an aside, deleting cookies and temp. internet files should be done as weekly maintenance
