Please reinstall Combat Arms. Therefore you have to go to C Drive > Program or Program (x86) > Nexon > Nexon Launcher
You will find there a file "Uninstall". This you have to use for unstalling Combat Arms.
After you have done this, please install the Combat Arms Launcher in complete fresh http://combatarms.nexon.net/downloads/gameclient.aspx
Go to your firewall and check that your firewall allows Combat Arms and set it as trusted.