Using the root grinder tip, got stuck. Won't move in forward or reverse gears. Tried to loosen manually, no luck. What can I do? And the machine is rented..

It is absolutely no fun when you get a cable good and stuck, huh? Hopefully the rental place has an employee that is very familiar with the machine and has run it many times. A person that is extremely familiar with the machine should be able to get it unstuck for you. You may end up calling in a plumber or drain cleaning company to get this resolved. Many times it takes a couple of good strong guys and bit of time and sweat to get these things unstuck. Or they may come in and pull it back right away (making it look easy). I have done more sewer augering with large power machines than I care even to think of. If you do enough, you are bound to get stuck every now and again. There really is no magic fix here. Determination and patience usually does the job here (not always). I have had to cut into pipes to retrieve cables only a couple of times, so lets hope it doesn't come to this for you. You will have to get the cable out one way or another, yes? Yes. After you do, I would suggest having a sewer camera sent down the pipe to see what you got hung up on. Please do let me know how things turn out. And most of all, good luck. Take care.

Great advice from Dad. City sewer or septic tank? You have a cutter tip,(see image) and not a auger tip. Right? How much cable did you have run out before it got stuck?
When that happens to me I put the machine in reverse and pick the machine up band pull back as hard as I can. Sometimes it helps when you jerk the machine back and forth when you're pulling. Good Luck. Tom