Oh dear

Not to worry, there's a few tried and tested tricks for finding lost little ones. Search the forum for 'lost snake', should turn up a lot as it happens all the time. Just a quick list:
Seal off all vents, holes, under-door spaces, drains that lead outside with something it can't prize off.
Don't just look on one level, these things can get right up to the ceiling so check the undersides of everything too.
Check everywhere warm, so boilers, top of fridge, underside of dishwasher, etc.
If you don't know which room it's in, sprinkle something powdery (flour or something) around all the entry ways so you can see if he slithers past and narrow down where he is.
If you think you know what room it's in, make some bottle traps (cut off end of bottle to make funnel, then tape it back the wrong way around to make it harder for snake to come out. Put pinky in bottle). Turn off the lights and leave the room, go back and check in couple of hours.
I'm sure there's loads more, main thing is not to panic, you'll probably come home one day to find it sat in the middle of the floor looking at you as if to say 'where have YOU been?!?'.