First, check the floor. JUST in case he jumped out, check all around the tank for him. It's highly unlikely, but there's always the IF.
Then, follow what they said on Fishlore.com. Put everything back in, and wait. You may not see him. Or you may have to sneak into that room at night with a flashlight and look for him.
I once had two kuhli loaches in a giant 150-gallon. I NEVER saw them. Then one day as I was draining the tank, I forgot to turn the heater off, and it melted the suction cups and dropped into the cold water. As soon as the heat and electric shock ran through the water, I saw one of my kuhli loaches come FLYING out of a piece of driftwood, and zipping around the tank! I felt SO bad for the guy - all other fish had been removed already and I forgot he was in there. I actually remember sorting through ALL of the gravel about 6 months prior looking for something, and I never saw him. So I assumed he was dead. Boy was I wrong!
FYI - He DID survive the electric shock and super-hot water, as well as the broken glass when the very hot heater hit the instant cold water and shattered. He lived for many more years after that incident.
