Even though hypnosis isn't really sleep, typically we describe returning to normal as 'waking up' from a trance.
It's probably the easiest thing about hypnosis. You simply give the suggestion that they will wake up.
The generally accepted best-practice is to count them up, leaving suggestions that they will feel wide awake, refreshed, and relaxed.
Typical end-of-trance suggestion:
"In a moment, I'm going to count from 1 to 5, and when I reach the number 5 you'll be wide awake, feeling relaxed and refreshed. 1, 2, 3, 4, 5 - Eyes open, wide awake."
If you don't wake them up with a suggestion, they might experience discomfort, disorientation, and sometimes even headaches. The best cure for this is to take them back under and wake them up properly.
For more information, check out our websites: iHypnoU.com