As there is little information to go from and I am not clairvoyant but I might guess you have a stock Motor possibly Brushed.
If that's the case I would say the motor either got too hot or dirty and the brushes are done.
If it's a sealed endbell you are out of luck with repairing the motor and it's time to upgrade.
I race toy cars for fun. If I need to explain, you'll never understand.