Try removing the drain plug in the block to see if any water comes out, you may have to poke it with some wire or a drill bit. I had this problem recently, nothing came out even with poking about with a long piece of wire. It yielded to a radiator flushing solution and I was rewarded with a brown sludge which eventually ran clear but Bernie is right it as it can set like concrete and I was probably lucky.
Cheers Richard
Richard Larter