My bearded dragon hasn't eaten for two weeks and just sits at the cool end

With a reptile that has stopped eating, the first suspect is almost always the enclosure, not the animal. A reptile that is too cold cannot digest food and cannot mount an immune response, so it simply switches off. Before anything else, measure the temperatures properly: a digital probe thermometer or an infrared temperature gun, read at the actual basking surface where he sits, and again at the cool end. Stick-on dial gauges are widely inaccurate and have misled a lot of keepers, and I would not trust the one on your glass. There must be a real gradient — a warm end he chooses and a cool end he can escape to — plus a proper night-time drop.

Then the UVB. The bulb stops producing useful UVB long before it stops producing visible light, so a bulb that still lights up can be dead for his purposes. Check its age against the manufacturer's replacement schedule, and check that nothing — glass, plastic, dense mesh — sits between the bulb and the animal. Exact target temperatures differ between reptile species; get yours from your vet or a species-specific care source, not a random care sheet.

Two concrete things. Weigh him weekly on a kitchen scale and log it: a cooling-related slowdown and real disease look completely different on a weight chart. And photograph the whole enclosure, lamps included, and take that photo to the appointment, because with reptiles the diagnosis is very often in the setup.

See an exotics vet promptly, rather than adjusting the heat and waiting, if he is losing weight, his eyes look sunken, he has tremors, twitching, wobbly back legs or a rubbery jaw, if he holds his mouth open when he isn't basking, or if he has passed nothing for a long stretch. Some species do eat less in the cooler months. Weight loss is never part of normal.