I was riding home today when my rear derailleur wouldn't up shift. I had to ride home stuck in first gear... It was too dark for me to stop and mess with it. I got home and after wiggling it around smacking it and what not, still stuck. It would stay in first gear only and upshifting just caused for a loose shifter cable. There was some grinding noise too. This happened suddenly right after a stop. I did not drop the bike or anything so this is definitely weird. Thanks for all the help in advance.