There is 3 main possibility causing this trouble; turning gear and solenoid valves in telegraph are actuated restricting flow of control air in the system. The air supply valve itself is shut or there is an issue with the pilot valve. Check whether the pilot valve is tank mounted, it is adjusted properly ( Valve differential adjustment ), pilot valve filters is not clogged, there is leak in the pilot valve UN-loader or something restricting flow of air to its inlet. You need to check for the above mentioned condition one on one and rectify the problem. I know its a hectic task and will take some time; so Good Luck!