Why wont your sand filter filter your pool?

I put water in the filter box and when I turn on the filter water is not circulating thru the chlorinator as if there is a blockage. I have cleaned both the areas and th skimmer basket and the basket to the filter are both clean. I have tried the waste cycle and the water is not escaping. Its very frustration as I generally use half a bucket to get it going and its now not going. Please help Regards Paul P