Hi when i start black ops and from the menu i select ZOMBIES it gives an error

Error

"The Call of Duty: Black Ops server is not available at this time. Please try again later or visit http:// callofduty.com/blackops/status for updates."

the same prblem is with MULTIPLAYER... I dont want to play multiplyer just tell me how to play zombies in single player//..
and when i start campaign it starts perfectly...