Simply a technician ;)About N2 lock out (or N1 that onground is more common)The engine to increase its performances needs that its blades rotates as near as possible inside the case, so the tip of each blade is very close to the border, any gap it can have is a loss of efficiency.A system called clereance control is able to cool the case that reducing heat expansion of the case and reducing the gap.In some circumstances, or failure, the difference between case temperature and the blades (including their disks and shafts) could create a gap that is 0 so the blades touch the case and lock the entire system.This normally happens if the engine is started and shut down without waiting the warming times.Another reason of shaft lock is a break of the bearings, the shaft breakage or some other. Another reason could be a FOD.The blades are mostly titanium alloy, they rotates at great speed, there are a lot of stages of compressor and turbine stages, all with a lot of blades, imagine an object that damages a blade, that object removes a piece from one blade, then, that object plus the piece hit the following blade, creating another 2 or 3 objects....It can be possible that this could cause an entire engine blades breakage, the engine will be empty and without blades air cannot make the shaft rotate. If I find pictures of damaged engines I'll post.

That is an example of engine blade damage, this damage is sufficient to stall the engine.Imagine that in the worst case all the blades you see can be entirely removed by the FOD or also for a single blade failure.An engine stall alone, (with no damage before the stall) could cause also blade breakage.

http://webcommunity.ilvolo.it/problema-ad-un-motore-md80-itali-airlines-t9070.htmlIt is written in Italian, but the pictures are sufficient to explain the possible damages.That is an MD80