When the engine can no longer rotate.
It could, any engine can "lock up" if not maintained properly.
The most common cause of engine lockup is a lack of oil. Also, an engine overheating from lack of coolant can cause an engine to lock up.
No, only the compressor will lock up.
Yes
Depends on what caused the engine to lock up. Most of the time a locked up engine is very serious and will require disassembly.
was there oil in it?
Lock up, No but a bad crank sensor will prevent the engine from starting.
Major engine failure?
the oil lubricates the engine parts, without it the engine would lock up and be ruined
Yes
Your engine could be locked up from low oil, burnt up rings or just a blown up engine