I had this problem too. I solved it by pushing down, but I was using the point and drag controls, so it was a bit annoying. I later found out that you can click the screen and get Guybrush to move down by dragging him.
Move the cursor around until you get a big circle on the screen with an arrow. Then hold the button down and drag down. This should get Guybrush back to where you can see him.