Try to run up the side of a bank that is meant to be a barrier. You may get stuck in a bush or behind some rubble or rocks. Rapidly tap X to jump while moving your soldier in side-to-side and forward motions towards the bottom of the bank. You will eventually find a spot where you can jump and get back on level ground. By doing this, you will not have to start over at your last checkpoint or restart the level to get free.