Open your antivirus and look for a "Virus Chest" or "Quarantine" section. If you see steam-api.dll there, select Restore and Add Exception Disable and Relaunch:
Locate steam_api.dll from a trusted source or another Steam game folder. black ops 2 cannot create steam-api.dll
Look for any recent blocks related to Black Ops II. If found, select the file and click . Open your antivirus and look for a "Virus