The fan on our downstairs unit is not turning now. I've tried turning it with a screwdriver but it's very hard and our upstairs unit (the working one) is very easy to turn, it actually spins freely.
How difficult is it to replace the fan motor? looks fairly simple.
If the motor is not damaged, you can oil the bearings and free the motor. spray WD40 on the shaft and allow it to run back into the motor end bell. Manually turn the blower as you do this and it should free it up. It's take a while to free whell. there should be oil holes on the end bells front and rear. Use 3 in 1 oil. If that dosen't work you will need a new motor. Any motor other than a OEM Lennox part will not last very long.