My first thought was rig up an air compressor to try to blow it out from the top. then I found this;
http://www.instructables.com/id/Un-plugging-a-sandpoint-well/
Give it a shot you've got nothing to lose.
It could always be the impellers giving out in your pump. Take the pump loose from the line and have it pump water from say a barrel. Does it pump pressure up quickly? Then more than likely the screen is clogged. Still pumps up slow? Then its your pump.
Won't be your tank waterlogged. All that does is cause the pump to turn on/off rapidly. It will burn up your pump motor btw.