Stuck in Minecart

I can not get out of the mine cart. Pressing L+shift doesn't do anything. Like other comments, I can mine below and around me but cannot distroy the minecart. Yesterday I saved and came back to the game to find I was finally out buy that isn't working for me now. Don't know if this helps but in third-person view, I find that I am crouched down. Maybe thats normal.