It is most likely either your internet connection, or the servers are having their maintenance.
Check
www.CombatArms.Nexon.net to see if the servers are temporarily offline.
Make sure your internet connection is working, and if all fails, restart your computer.
If none of the above works, you're best bet is to either wait it out, or if you know something's wrong with the files of Combat Arms, then you should re-install.