I have had this happen on my Daysailer and my 222. On the
daysailer, one of the centerboard control lines got wedged between the board and trunk. The boat was on the the trailer , and I was able to pry the board down. On my 222, I had ran aground in soft sand with the board up, I put the boat on the trailer for about a month. The next time I tried to lower it, it wouldn't budge. With the boat in the water, I took a screw driver under the boat and pryed it down. Once it moved, sand was falling from the trunk and the board fell down. From then on, when I run aground, I will lower the board to clean out the sand. No problems since then. I think you will have to pull the boat or get wet.