Use ControlPanel to completely remove CombatArms(if you still have it)
Then go to your computer and find the Nexon/CombatArms directory, if there's still a folder called CombatArms, delete it.
Now re-install CombatArms it should work. Installer or normal save>run>install.
Btw, there's a help section so post it in the right section next time.