Hi there, and welcome to the forum!

Frogs can be little Houdini's sometimes!

Even treefrogs sometimes bury themselves in their substrate if they're stressed or feeling unwell. If you can't find the frog hiding in any plants, it's a possibility he's buried himself.

If you need to find him, VERY GINGERLY/CAREFULLY start to loosen the substrate, and try to locate the frog. If you haven't had him for long, he's probably very scared and stressed from being in a new home. When you find him, ensure he has lots of plants to hide amongst, and that you cover three sides (the back and two sides) of the terrarium in dark material (preferably dark green) to make him feel more secure. Also try not to handle him for a week or so

Good luck, and keep us updated!