Are you saying you found a regurged mouse in the cage???
First off...if he was "gone" and then regurged in the cage and then "disappeared" into the hollow branch again...he's not stuck. ;) It's extraordinarily rare for a snake to get stuck in anything. Their flexibility make human contortion artists look like 400 pound grandmas!
Second off...you'll need to try and figure out why he regurged and help him to not do so again. Perhaps just the stress of you flipping through the cage the way you have been when you thought he wasn't in there. You'll want to skip his next scheduled feeding in order to give his digestive system a chance to heal and right itself. Snake regurges are very hard on their system and feeding them too soon can cause a chain reaction of more regurging.
VERY glad to hear you found him!!