I am going to assume that your Exchange environment consists of a single Exchange server and is also your AD box. Don’t check for strictly logon issues as there could be something wrong with AD and/or your ISA. Personally, I stay away from ISA unless as a last resort. Get a real firewall in there.
I would also double check that the POP3 service has not stopped unexpectedly and is set to automatic start up. Also check to make sure individual accounts still have permissions for POP3.