I had a similar experience on 2 different occasions. do you have mods enabled? If so disable any that apply to script changes, and or Movement. Enable after the full into, after exiting the cave. In mine Helgan's walls were gone, and the cart in front of me hit a bump, then went spinning into space, then my cart did the same thing. The other time i never got to the block just stood there waiting for them to push me down.