The easiest fix is probably just to depressurize the keg and pull out the "out" diptube. Hold it up to a light, and see if it's clogged. Make sure you clean out the poppit too. That will fix it. I've seen "diptube screens" for sale, but I don't have one. Put it all back together, and if you have a stainless scrubby (like SOS pads without the soap, and it has to be stainless!), sanitize it and put it at the bottom so that the long diptube goes into it. That should help.
Broken Leg Brewery
Giving beer a leg to stand on since 2006