I just wanted to clarify you are referring to Black Ops II and not Black Ops I. Is this correct?
Please specify the platform you are on as well as the connection type (wired/wireless). As nuttin2say indicated, the NAT type is important information to know as well. Please let us know the NAT type that is displayed in-game.
Also, let us know how often you are getting disconnected. Was it a on time occurrence on that particular day or does the disconnect happen quite often?
In the meantime, please try rebooting your router, modem and system. You may also consider establishing a establishing a static IP address and port forward.
Here are the port numbers you will need:
Xbox 360: Activision Support
PS3: Activision Support