Establish the problem that you are facing with the motor before determining the line of action that you will take. The motor may fail to start, overheat or make a lot of noise than expected.
Make sure that the power is off before checking the motor, as this could pose a serious threat if on. If the motor fails to start, check whether the line fuse involved is blown or whether the circuit breaker tripped. If not, give the motor some time to cool before resetting. However, if the line fuse is blown, replace the fuse with a viable one in the fuse box. Use a screwdriver to open up the fuse box and simply yank out the blown fuse to remove it.
Check whether there is anything clogging the motor shaft from spinning such as dust and other foreign particles in the system. Remove these hindrances using a brush or a blower where necessary then try and start the motor again.
Rotate the rotor slowly with your hand to see whether there is any resistance to prevent it from spinning smoothly. Open the motor casing using wrenches and screwdrivers to expose the internal components and check whether the shaft is bent, not aligned properly or whether there are any damaged bearings. Then, replace the damaged components. Check whether there are any loose mountings and tighten them up once more using wrenches and screwdrivers. All these defects could lead to excess noises in the system.
Replace the extension cord with a thicker or shorter one if the motor tends to overheat. Check and unblock any motor holes blocked in any way and make sure that the fan is working properly. Replace the fan as well if it isn't and this should make the motor work properly once more.