Did you crash recently? The drive shaft in these quadcopters breaks fairly easily. If you find that the motor does not spin freely by hand, you'll need to replace either the whole motor arm, or just the motor itself. Replacing just the motor is more time consuming, but you'll be able to find parts for a lower price. [https://www.bhphotovideo.com/c/buy/udi-rc-accessories/ci/28513/N/3750554691|B&H sells both motor arms and individual motors|new_window=true].
When purchasing replacement parts, '''be sure to triple check whether you need clockwise or counterclockwise motors,''' as using the wrong ones will prevent your drone from flying.