In most cases, the spray gun is not the problem. The problem is the rubber sleeve covering the fitting at the end of the high pressure hose. It has likely slipped up toward the metal tip.
The fix: wiggle, twist and/or pull the rubber sleeve down until the entire tip is visible plus an 1/8” of the metal fitting below the tip.
Now try to insert the hose tip into the spray gun. It should “click” into place and will not come out until the hose is released by pushing in the lever on the underside of the gun.
You may cut away the end of the sleeve with a sharp knife if unable to move the sleeve down. Be careful not to cut or puncture the hose.