If you find that you cannot attach the gun to the hose, the cause may be a bent u-pin inside the gun.
When you look into the gun where the hose is inserted, by moving the release button you will see a locking spring pin that will lock two sides of the hose connector into place.
By using a small flat screwdriver inside the cotter pin, you will be able to press the spring pin towards the center of the opening.
Do this similarly on both sides and you will have restored the shape of the attachment for the hose.
The coupling and attachment AVA uses is a well-established standard coupling used by most brands today. This makes it easy to change the o-ring, which is a wearing part, and it gives room for movement in the gun (swivel), while the disadvantage is what you have just experienced. You will probably not experience this again if you are careful to always release all pressure from your system before attempting to release the gun.