What you're describing is usually referred to in junkie slang as "falling out". It's happened to me a few times within the past months, and you usually just wake up and don't even realize it happened.
If it happens when you're with somebody I would recommend monitoring their breathing, making sure they don't choke on their spit (or puke). They will usually wake up within 30 minutes.
When I used to be a junkie we would always carry suboxone on us and a syringe close by in case anyone ever overdose hardcore (not breathing) . An IV injection of a very small dose can wake them up and save their lives .
Don't take this as medical advice, take it as personal experience.