The answer is yes: if one of the pilots needed to use the restroom, regulations require the other pilot to put on his O2 mask while the other pilot is away. Once outside the cockpit, the other pilot could lock the door and the other pilot out. Then he could turn everything off.
Also since it was at about 1:30am, most passengers would be trying to sleep in their chairs; additionally the masks that drop only have enough O2 to last while the aircraft descends to a lower/safer altitude — so if the plane maintained altitude, there would only be 5-10 minutes of O2 and everyone would pass out and eventually die.
Unless it landed on smooth water, like the miracle on the Hudson, the plane's ELT (Emergency Locator Transmitter) would go off and the crash site would be pinpointed by satellites within seconds.
So therefore one of two things happened:
- It landed on water softly and is now at bottom of ocean, or;
- It landed safely on land because there was no ELT signal.
Nothing else is possible. The passengers are all dead from hypoxia; I believe it may be in Indonesia, or another Islamic nation in the area.