Whatever it is, it will be expensive to fix if you have to pay a shop to do it. Just how expensive depends on what has locked up the motor. For example, if a head leak has the engine hydrolocked, then you might get away with only needing to replace one head gasket (out of the two). And if its the right hand bank (front of car) this is easier than the other bank. Or you might find that you’ve had a head leak for a while and this has damaged other things and the whole engine ought to be rebuilt. Or you might find that it’s not a head issue at all but a lower end issue which would also require a rebuild.
No one can tell you whether its worth re-doing. That’s a question about you and the rest of the car. You get a guesstimate about what it will take you to get back on the road and then ask what kind of car you’d be able to buy for that amount of money. You also ask yourself about what is coming up soon for this car. Will it soon need tires? Brakes? Suspension work? Then see what you think.
I would first double-check that engine won’t turn over by hand. If not, I would start pulling spark plugs to look for coolant in cylinders - just to get an idea of what is going on.