Ugh, that's always hard to hear. If you haven't already, take apart the tank. If there are any decorations your newt could crawl inside, check those with a flashlight. Otherwise, look around every little piece of furniture, every decoration, any hole it could crawl behind or into, especially if they're dark, damp, cool, or humid. If you don't find him soon, leave little trays or plates of water lying around the house and check them every hour or so, and look anywhere there's water, like the bathroom. Depending on the climate, he should be able to survive 8 hours to two days without water. (This is why the plates are important.) Good luck finding him- keep us updated!