Yep thats dayz for ya everybody seems to be having similar problems.
You can try this it works for me most of the time
Go to:
C:UsersYOUR COMPUTER NAMEAppDataLocalArmA 2 OAC
and delete this folder:
OAMPMissionsCache
note the AppData folder is hidden and you have to change folder options to *show hidden files, folders and drives*
incase you dont know- go to control panel, then theres *folder options* then in the view tab its aboot the 6th one down