Uh, that is perfectly normal horned frog behavior. They are definately Not active frogs; they can bury themselves for weeks at a time sometimes. Mine does this, and only comes out after a defecation, and then she'll usually just bury herself in another spot in the tank.
It would help to keep the substrate moist (NOT sopping wet though) so the frog does not dehydrate while buried.
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2.1 Ball pythons: Goblin, Nothing, and Bela
1.0 Boa Constrictor Imperator: Apache
0.1 albino Cranwell's horned frog: Bene
1.0 Tiger salamander: Slasher
1.0 black kittycat, Inky
A bunch of Oscar cichlids, one giant pleco, huge breeding lot of "fancy" (read: deformed) goldfish, and me an' the boyfriend.