"Hydraulic lock" is what happens when there is excess fuel in the combustion chamber and it stops the piston as it goes up towards the head button (liquids do not compress). If this happens while trying to start your engine, carefully remove the glow plug and turn the motor over a few times to get the excess fuel out.

To keep the fuel from building up in your engine while it is not running, here are two good preventative measures to take.
  1. Always empty the fuel tank when working on the car. While the car is getting moved around, lifted and flipped, fuel can easily siphon into the case of the motor and causing trouble next time you try and start the engine.
  2. Always keep the carb higher than the fuel tank when it is not running. Especially when you are moving from the work bench to the pits. I can't tell you how many times I have seen pitmen try to start the car just before the start of a race and the motor is hydraulically locked.