[Lynn Eades]

If you can, tip the motor over so you have one bank of cylinders pointing up. Pour some diesel into each pot, and set it on fire.
Let it burn.
The heat will expand the componentry, and hopefully free things up. It will take a while.
Do the same to the other bank, and then try barring (turning) it over.
This is for when the rings/pistons are stuck. If you have stuck valves, I would deal with them first. If you think the piston pins are stuck, roll the motor over with oil in the sump to saturate them.