How do you unlock a engine?

When you say a engine is locked, many scenariOS can come into play. If the vehicle has been running recently and has A/C, make sure the compressor is not froze up as the starter will not turn over an engine if this is the case. You may have to remove the belt to verify this. In all other instances, first remove the spark plugs. By doing this you are reducing compression or resistance. If the starting system is up to par, (battery, starter) disable the ignition system and try to crank the engine. If not try to rotate the crankshaft with the appropiate socket and ratchet or breaker bar. If it rotates, notice if fluid (water or anti-freeze comes out of any spark plug holes. If it does, then the engine was hydrolocked. Air can be compressed, but water cannot. Meaning a bad head or gasket. If engine doesn't rotate and has been sitting for a long time (as in years), then pour a small among of oil into each plug hole to help break any rust buildup. Some people recommend using PB Blaster spray. Some others even say to use brake fluid. This procedure may require an overnight or 2 of soaking. If you do get the engine to turn but only partway in either direction, you may be looking at some serious broken part inside the engine which in turn will translate into disassembly to find the culprit. Hope this helps you.