The first thing I'd do is remove the serpentine belt, then try to start it. And if it does start, don't run it for long because the water pump won't be spinning, but you've determined it's something in the accessory drive and you can narrow down from there.
And if it still won't start, what does it sound like? If the engine really is seized, there should be a pronounced noise as the starter engages the flywheel that won't rotate.
Next I'd take the spark plugs out to see if it will rotate that way. Could be hydro-locked. Just guessing, but it's very easy to rule that out.
Finally I'd put a socket on the crank pulley bolt and try to turn it with a long breaker bar. If the bolt starts to rotate further in, STOP. It's time to pull the engine.
And if it still won't start, what does it sound like? If the engine really is seized, there should be a pronounced noise as the starter engages the flywheel that won't rotate.
Next I'd take the spark plugs out to see if it will rotate that way. Could be hydro-locked. Just guessing, but it's very easy to rule that out.
Finally I'd put a socket on the crank pulley bolt and try to turn it with a long breaker bar. If the bolt starts to rotate further in, STOP. It's time to pull the engine.
