Hi, sorry to hear that your beardie isn't eating. How old is he and how long have you had him? What had you been feeding him when he was eating? Had you been dusting his food with calcium and vitamins?
In order for us to be able to try to help, we need to have some info about his tank setup. What size tank is he in, what is the wattage of the basking bulb, what are the tank temps, what type of thermometer are you using to check them, what is the brand & type of UVB you're using, how old is this bulb, what is the distance of this bulb to him? How have you been keeping him
hydrated? What are you using for a substrate on the bottom of his tank? Is he pooping daily or every couple of days? Have you had fecal checked for parasites? Is he housed with any other beardies?
The quicker you can answer all these questions, the faster we can try to help.