I don't know what your full setup is like so I'll just hit the basics.
If you have an air pressure control then someone may have turned it all the way down or the control may have gone bad.
If you have moisture traps then you may have a plugged filter.
If you have quick disconnects built up sediment or rust could be blocking the auto shut off.
If you don't drain your tank regularly moisture can form an ice ball that can temporarily block the air.
There's not really anything else on a basic system that can stop the air from getting out of the tank.