You can always check online services status here Call of Duty: Black Ops 2 Status
If online services are fully operational and you are still unable to connect, you may want to try the following.
- Reboot your router.
- Try using a wired connection in order to sustain the connection quality.
- Establish a static IP address and port forward.
Here are the port numbers you will need:
Xbox: Activision Support
Should you have any questions on how to port forward, please contact your ISP or the router manufacturer.