If it hasn't been in the water for 3 years, it's not likely to be anything (mud, stones) in the case. So more likely to be the uphaul system. My earlier one is different, but make sure the keel is supported - there may be a wire strop, or dry out somewhere - then loosen the uphaul rope and see if it runs through the pulleys ok; it may have jammed somehow. Or, depending on the type of rope, if it's had tension on it for 3 years, it may have developed a permanent bend which needs straightening.