We call it the "sinking mind". Often this comes because the concentration is in excess of awareness. Sometimes you can't come out of it until it ends by itself, but sometimes when you start to see that you are sinking into it, you can try to bring up the awareness level by being more aware of the changing nature of whatever is happening. For example, sometimes when we're watching the breathing some people just absorb into the rhythm of the breathing. So in order to get the awareness level up, to make sure that the concentration doesn't get so strong, we have to try to notice the changing nature of the breathing. Is it long, is it short, is it rough, is it smooth, is it changing? How is it changing? How is it changing with the different states of mind? This helps to bring up the level of awareness.