Here is the error message in its entirety:
Failed:(-1073548784) Executing the query "BACKUP DATABASE [MCM] TO DISK = N'F:\Backup\MCM\M..." failed with the following error: "BACKUP DATABASE is terminating abnormally.". Possible failure reasons: Problems with the query, "ResultSet" property not set correctly, parameters not set correctly, or connection not established correctly.
There is no further information. No backups for any database on the server are working right now, but they did succeed the day before. There is plenty of backup space. One thing to note, though I'm not sure if it's relevant, is that there was a transaction stuck in a Killed/Rollback state the day before; it was blocking on a remote procedure call to a Sybase database. The service was restarted to try to "fix" this stuck transaction (by someone I warned beforehand against doing such a thing).
What do I need to do to fix this situation so that I can get the backups working again?