Squeeling noise coming from AC, compressor is on, fan not turning
The squealing noise is coming from the compressor. This noise is common when the unit reaches extreamly high pressures. The pressure is most likely high because the fan is not running. Since you can spin the motor it could be that the motor has damaged windings. Leave the unit off until the repairs are made to ensure no further damage to the compressor. You will need a multi meter and some experience reading wiring diagrams to properly diagnose the system. You can perform an amp draw on the motor to see if it is in spec. You can measure the volts to the motor and see if they are in spec. Finally the contactor can be measured with the proper meter to see if it is working properly.
My suggestion is to have the Motor and capacitor both replaced. Also inspect the wiring in the condenser electrical compartment for any signs of damage or burning.