Well not being able to screw in the gas tank is a seperate issue. First lets look into that so we can diagnose it further.
Have you tried any other air sources? Some of my older guns dont like newer tanks. On those guns I need to toss in a Oring into the ASA itself before putting the tank on. Also only turn a quarter turn past when you hear the tank air up any more and you risk damage to the tank.
As for the gun not firing Are you sure its air pressure was good? If it has leaked through a small hole or bad oring that might be the reason why.