Assuming you are talking BOTH Black Ops games. What error message are you getting on each game ? Are you sure you werent banned ?
Otherwise try these one at a time in this order.
Reboot your network.
Clear your cache.
If the above dont work, Delete ONLY your gamertag not the saved games, then redownload it.
Try also playing Combat Training and tell us if that works.
I realize its been a couple of days, but wanted to reach out and see if you are still encountering this issue? Just let me know and I will do my best to troubleshoot with you!