My arma II games disappear ;(

Just wait for an answer from Steam support.

If you got the copies via trading, the person may have used a stolen credit card (in which case, all purchases made with it are deleted).

Steam will contact me without my first word?

jak kupujesz gry na allegro to chyba moze ci ktos potem je odebrac cos tkaiego slyszalam

I think you can ask for a refund for the game you used to tade for it.

steamswallowed

Check the game's store page it might have been uninstalled