Here are some things I would check:
1.) Make sure that rebooting the server in that instance didn't cause any errors/changes on the server end - if possible.
2.) Check her pop settings to verify they are correct, in case she possibly changed something while in panic mode of not receiving emails.
3.) Does your server have any kind of password lock-out option, or is there one on her end, so that maybe she tried a few times, and it didn't connect, but therefore blocked her access to it?
4.) Try creating a new account in her outlook, and see if the new one works... if it does, delete the old account.