Despite being desert-dwelling animals in the wild, bearded dragons should not be kept in enclosures with sand (or other particulates) as a substrate. They’ll ingest it with their food, and it can get stuck in their digestive system and can kill them.
They enter a hibernation-like state (called brumation) typically once a year, and it can last up to 3 months or so. They stop eating and spend most / all of their time asleep in their hide, and if you’re not expecting it, can look like they’ve died. It’s normal, healthy, and they’ll wake up and go back to normal again.
Bearded dragons are frequently carriers of salmonella, which they can pass on to humans. Always wash your hands thoroughly after handling one.