Pull all of the plugs, and then try it.
How many miles?
My wifes 98 Lesabre died on her right around 100,000 miles. The plastic intake manifold on those melts where the EGR pipe comes into it, and antifreeze leaks into the intake and hydro locks the motor.
If that is what happened, you'll be able to turn it over with the plugs out.
Whoops, I should have read it more closely. If the rockers aren't moving, then you probably have a broken timing chain.
