Arduino DC Motor stuck, cant even rotate by hand

[notenlish]

I have a mini sumo robot kit and I've assembled it, everything was working fine but for some reason one of the dc motors got stuck and I can't even rotate the motor by hand anymore. The power supply is 8.4v 300 milliampers. I've found a post about a dc motor not working like mine but couldn't understand it.

The kit I'm using: https://www.robotzade.com/urun/senju-mini-sumo-robot-kiti (My power supply is different than the one the kit uses though.)

[notenlish]

[Paul_KD7HB]

That tells me the brushes in your motor are worn out and have fallen out of their holders and jammed the rotating part. Either the motor was worn out before you got it or was not properly built. You can't fix it and will have to get another motor.