Sorry to hear that. Unfortunately I'm not much of an Arma expert. You might get more (and quicker) help in the Arma forum here on in the official DayZ forum, lots of helpful guys there.
EDIT:
Found this, maybe it helps:
Close Steam completely.
Find the Steam.exe in your program files and run as ADMINISTRATOR.
Log in to Steam and launch ARMA 2: OA.
Exit out of that game and then load ARMA: CO.BTW the increased Zombie numbers really make for some interesting shopping trips into the cities...

Edited by Para_Bellum (05/14/12 07:14 PM)
_________________________
"...late afternoon the Air Tasking Order came in [and] we found the A-10 part and we said, "We are going where!? We are doing what!?"
Capt. Todd Sheehy, Hog pilot, on receiving orders during Operation Desert Storm