DIAGNOSIS Your thermostat isn't getting power. The most common cause for a completely dead T9 is the C-wire (common wire) either not being connected at the thermostat or at your furnace/air handler control board.
FIX First, pop the thermostat off its wall plate. Check the bundle of small wires. You need to see a wire connected to the terminal marked 'C'. If there isn't one, you'll need to trace that wire bundle back to your HVAC system's control panel and ensure a wire is connected to the C terminal there. Often, there's an unused wire tucked back in the wall you can use. This is a low-voltage job, but if you're not comfy opening the furnace panel, call an HVAC tech.
IF THAT DIDN'T WORK If you have a C-wire connected and it's still dead, the wall plate itself might be faulty. Try reseating all the wires to ensure they're snug, or contact Honeywell support for a replacement plate/transformer.
HEADS UP Don't just jump the R and C wires at the thermostat to test—you can blow the fuse on your furnace control board. The fix is usually just pulling that little 3-5 amp car-style fuse on the board, but it's an avoidable headache.