That depends. Did you fall from the spot where the Mako is parked? If so, you have to find a way to get back up to it. If you find yourself unable to do that, you should revert your save to a position where you were IN the Mako or to go back to the Normandy. If you are approaching the Mako and press A (assuming you have the 360 version) or E (assuming you haven't changed the deafult settings if you have the computer version) and you cannot enter the Mako, you are probably either subjected to a glitch or you should move closer to the vehicle.