Looking at the dramatic Daily Mail story of passengers on an Easyjet flight to Malaga, fearing that they would die, as it, "Plummeted" 20,000 feet, here's why the Oxygen masks in the aircraft, releasing in the passenger cabin, shouldn't provoke immediate panic.
The masks will appear automatically if the cabin pressure drops below a certain level. Without that necessary pressure, which is normally maintained at around 8,000 feet, your body won't be able to use the Oxygen in the air to stay conscious, which is why high-flying passenger aircraft are equipped with Oxygen masks, to deliver a higher proportion of the vital Oxygen your body requires for the period of time that the pilot will need to take the aircraft down to 12,000 feet, where you won't be struggling to breathe.
So, if there's a sudden pressurisation problem, the pilot will try and take the aircraft to a safe level as rapidly as possible in a safe but very rapid descent, which is uncomfortable and worrying for the passengers but is not dangerous.
This is a very rare occurence, given the millions of passenger flights that take place but if it ever should happen to you, now you know.