In order to play "Call of Duty: Black Ops" on the PlayStation 3 gaming console, you need to connect to one of the servers at Treyarch via the PlayStation Network. If you continue to get a "Server Error" message every time you try to access a lobby, this is an indication of a problem with either your copy of the game or the system itself. Troubleshoot these issues by looking closely at the console.
Verify that your console is logged into the PlayStation Network. If you're receiving a "Server Error" as the game is trying to connect you to a public lobby, your console may have become logged out of the network. Return to the menu on your PlayStation 3 and select "PlayStation Network." If the words "Sign In" are on screen, you are not logged into your account. Select "Sign In" to sign in.
Check the official Sony PlayStation Network "Status Update" page to see whether the PlayStation Network has gone down. The PlayStation Network periodically goes down for routine maintenance, which gives you a "Server Error" message every time you try to connect to "Call of Duty: Black Ops." If the network is down, you have no choice but to wait for it to come back online before you can play.
Delete the "Call of Duty: Black Ops" installation files from your console and reinstall them. The "Server Error" issue could be an issue with the original files installed the first time you played the game. Select the "Game" category from the PS3 menu and then select "Game Data Utility." Highlight the "Call of Duty: Black Ops" icon and select the "Delete" option. Put your game disc back into the console and let the game automatically reinstall itself.