Being conscious means generating thoughts (citta).
We lose consciousness (i.e., do not generate thoughts) under the following conditions:
– Deep sleep
– While being unconscious either due to a traumatic injury or under anesthesia.
In addition, an Arahant can DELIBERATELY get into nirodha samapatti for up to 7 days, where again no thoughts are generated.
– In order to get into nirodha samapatti, one must go through the four rupavacara jhana first, and then successively go to higher arupavacara jhana up to the highest, the “neva sanna na sanna” state. From there, an Arahant can make a determination on how long to stay in nirodha samapatti and get into it. (other yogis may be bale to get to the “neva sanna na sanna” state, but CANNOT get in to nirodha samapatti).
– Just like while being unconscious in the above mentioned “mundane situations”, the physical body of an Arahant in nirodha samapatti is kept alive by kammic energy.
Attaining Nibbana (or the Arahanthood) does not mean losing thoughts. One just will not generate any DEFILED thoughts; this is also called saupadisesa Nibbana.
– That is called “vinnana nirodha”, where vinnana is a defiled mindset, where defiled thoughts may be generated.
– However, when the physical body of an Arahant dies, another birth in the 31 realms does not happen. Thus thoughts (citta) cease to exist PERMANENTLY and the Arahant merges with the Nibbana dhatu (element) or attains anupadisesa Nibbana.
– Two types are Nibbana were discussed here: “Reply To: Nibbana, not of this world?“.
I think that summarizes all possibilities for “ceasing consciousness”.