Sometimes when you first pour a beer you get to much head and it appears the keg is over gassed. This is because the tap which is outside the fridge is warmer than the beer in the keg and when the beer passes through the warm tap the beer foams up giving the appearance of being over gassed. Once you have poured a couple of beers this problem usually goes away.
If your keg is over gassed you will need to purge some gas out. First thing is to turn off the gas to at the bottle and simply purge the gas through the release valve until no more gas comes out. Wait about 10 minutes and pour a beer. You will often find the gas pressure in the keg has built up again and there will be enough gas to pour a beer without turning the gas on at the bottle, you may have to repeat this process a number of times depending on how over gassed you keg is. If no beer comes out when you try to poor a beer turn the regulator down to about 5psi/35kpa and try pouring again.
If the keg is still over gassed you may need to leave the release valve open for a number of hours or even overnight. Sometimes when you do this you end up releasing too much gas and the beer ends up flat. If this happens don’t worry just hook the gas back into the keg and set the pressure at 40 psi (280kpa). Gas the keg for about an hour and leave it sit for about 4 hours to absorb the gas. Then set your regulator back to the pour pressure and try again. Remember there is not hard and fast rule to degassing a keg so it will be a bit of trial and error. Good luck