The screen went blank on our Honeywell T8600D Chronotherm IV Deluxe Programmable Thermostat. I have pulled off the cover and the copper connectors look fine. This happened a couple weeks ago, just pressed one of the buttons and it came back on. Not this time. 20 degrees out so would like to get it functioning! Yikes!

If battery operated replace with known good batterys. Also check for 24 volts at your unit to be sure thermostat is being powered. If all of that checks OK then it might be thermostat time.

Mystery solved. Husband was lifting weights in the furnace room and hit the switch on furnace, turning off power!

LOL Now that is a good one for my list of things to check. Thank you for posting back. You figured it out yourself so that rates a bonus.