Another possibility is that the nylon block at the bottom front of the keel housing has split, and the keel's jamming in it (happened on my P21). To see if that's the case, try looping a rope round the front of the keel under the hull and back to the 2 sheet winches. Put a bit of tension on to see if the keel pulls back (only an inch or so) and then raises ok.
Like Ken, I just made up a replacement block from a bit of nylon. A bit fiddly to take off and put back on though!