Hi, I did a search on here and couldn't find anything like my situation. So the other day I backed my mountain bike out of my garage unitll the back wheel locked up then when I went forward it was fine. I rode it up the road a bit and the gears or cogs were skipping eventhough I wasn't changing gears. The cable seems to be tight and I think I remember someone telling me never to back up a mountain bike. The rear derailleur also looks like it is too high up. What is wrong and how do I fix this?
It sounds to me that you may have over-rotated the derailleur.
Where the derailleur mounts to the frame, there is a flat metal bracket that has a screw, the "B-tension" screw, and a small tab bent at a 90 degree angle. As the derailleur knuckle rotates, the tab butts up against a lobe on the knuckle and prevents the internal derailleur spring from rotating past the lobe...most of the time. Tangling the chain by backing up the bike while a gear has been shifted but not yet moved to the next gear can bend the tab and allow the tab to slip past the lobe. If it is not too damaged, you can often rotate the derailleur back, and with a punch, peen down that tab again until it holds.
That is exactly what happened when I backed up the bike. The chain got all tangled and the derailleur is unaligned. I am still working on it and think I can get it up and working now, and thanks again for everyone's help.