Your adjustment for the shifting cable may have vibrated out somewhat. You might try disconecting the linkage and try shifting it by hand. If it goes fully into reverse, all you need to do is readjust the linkage. You can do this out of the water as long as you are feeding water to it. Walmart sells a plastic clamp with rubber suction cups on both sides. Hook that up to a garden hose, clamp it over your water intake and turn the water on. You can safely start your boat motor. Remember to stand clear of the prop while it is spinning. Also, make sure there is water coming out of the "Pee Hole" and try not to rev the rpms too high for very long. Leave the water running until AFTER you have shut your motor down!
Good Luck!