Ubisoft's said in a tweet that the DRM issues that's affected titles like Assassin's Creed II and Silent Hunter 5 for PC were down to a "server attack" last night.
Because of that, "limited service" was only available for both games between 2:30pm and 9:00pm CET.
"Apologies to anyone who couldn’t play ACII or SH5 yesterday. Servers were attacked which limited service from 2:30pm to 9pm Paris time."
Update: Ubi just issued a statement on this:
“Ubisoft would like to apologize to anyone who could not play AC2 or SH5 yesterday. Servers were attacked and while the servers did not go down, service was limited from 2:30PM to 9PM Paris time. 95% of players were not affected, but a small group of players attempting to open a game session did receive denial of service errors. All player with an open session during the attack were not affected. We also confirm that, at this time, no valid cracked version of either Silent Hunter 5 or Assassin’s Creed II are available”
Assassin's Creed II released on Friday for PC in the UK, with it entering 14 in the UK charts released this morning.