There's a gremlin in the system it would seem, after the online portion of Dark Souls: Prepare to Die Edition has mysteriously gone missing on PC.
The multiplayer part of the game went offline about a week ago, around the same time as Dark Souls III launched here in Europe, which may or may not be a coincidence (although it's certainly conceivable that the two events are linked).
There's plenty of theories flying around, but ultimately nobody outside of Bandai Namco and From Software knows the specifics, although a US-based community manager took to Twitter to say: "Dark Souls PTDE Matchmaking. Not sure why it's being funky. Have made dev team aware - Will share any updates."
According to Kotaku the PS3 and Xbox 360 versions are unaffected, so for the time being it's just PC-based players who are having to go it alone in From Software's notoriously challenge action-RPG.