Starter could be bad, neutral safety switch, wiring, engine could be locked up.
It means that there is something wrong with the engine and not the battery
It could be anything from the engine to the connectors
chances are the battery is out of wack or the engine could be flooded
More than likely the battery is to weak to start the engine. If it's not the battery, the starter may be the culprit.
Somthings wrong with the engine.
There could be a sensor wire that is bad or going bad
There is the possibility the battery could blowup. You can damage the complete electrical system.
Faulty Alternator?
It means your battery is dying
Never disconnect a battery while an engine is running!
Try a new battery, or get a jump start. Sometimes there is not enough power in the battery to start the engine.
Probably a dead battery.