2
Got any recent backups for the files?
(if not you might need to download them all and do a search and replace over all files to get rid of it. You can try to user
Powergrep if necessary it has a 30day testversion so you can check it out)
Contact your hoster to see if this is a general problem and affects other sites on the server.
Might be some security hole like php version not up to date on the server for example.