The Pull cord was jamming. I took it apart and

Either the motor is frozen or the start assembly is jammed.

Pieces of the carpet jammed the brush, if you can't pull them out from the front you will need to take the head apart to clear the brush.

you wrapped the rope backwards

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One or more of the bushings is seized, the machine needs to disassembled, the bushings removed, cleaned, re-lubricated. Not something you can do yourself.

It has wrapped around the motor pulley causing the machine to lock up. You will have to get into the motor to remove the cord. This may have also broken the belt.

There maybe a split pin holding the cover down over the spindle! Not too sure, but possible. Could be pushed down the centre, just a case of having a good look. Only really worked with Hurqvarna's and Makita's. Hope it helps. Good Luck
RICH

You need to check and see if it broke a sheer pin when it jammed,they look like reguler bolts and can be found in the tubeing that you see when looking at roto in the front.

I had this problem with my Roomba Discovery. I ran across your question while searching for an answer. My main symptoms were, Roomba hesitated a lot, and the dust bin did not fill. I had plenty of hair wrapped around the brushes and against the filter.
I just finished taking my Roomba apart. I found the motor drive shaft had a LOT of hair wrapped around it and it had climkbed into my gearbox. I took that apart and cleaned all the hair out of the gearbox. My Roomba is back to normal now, filling the dust bin.

you need to replace your shukle that holds the tread bobbing below.it is either slack or broken . if it is slack then you tighten the little screw that holds a cover that gives the thread tension .if its broken then replace it.once the thread is slack below it will cause your thread to bundle up.