At the top of the stairs is what looks like an impassable pile of rocks, but if you start at the bottom of the stairs and rev the engine before taking off, you can use the stone ramp on the left to jump the motorcycle over the rocks. You rev up by holding down the brake and giving the bike gas.
Ok...well after you pick up Treasure 44/50 you get back on the bike and continue to the next cave in, which is a short distance ahead. There you should find the Xibalba Key, which is what you need to get.