My central air outside unit fan humms but won't spin what do you think the problem is

My central air outside unit fan humms but won't spin what do you think the problem is

You have an outside fan that's seized up and needs replaced. Assuming you don't have a tree branch lodged in the unit preventing it rotating.

Two things to check-- Run capacitor for the fan motor is bad and the motor can't start or the motor is bad. You can take the capacitor to a local hvac dealer and they should test it for free, in hope that you might buy the part from them.