Pull the distributor cap off, use a vacuum pump brake bleeder etc, hook it to the vac can and apply vacuum, If is doesn't move there is a problem which must be further investigated.
Next step would be to remove the vac can and see if the plate will rotate by hand.
If you take the VAC can off the distributor the plate should rotate freely.
BTW if you don't have a vac bleeder etc I have pulled another car up beside and used a long piece of hose.