Honeywell chronotherm IV Plus thermostat. I have a honeywell chronotherm IV thermostat. For some

Question

I have a honeywell chronotherm IV thermostat. For some reason it seems to have stopped working sometime this afternoon. (the screen has gone blank)

I tried flipping the fuse box and it did not help. I am looking to see if there is perhaps a battery that I can replace but have not yet seemed to locate one. I don't believe it's the heating system in the house as the heat has been working properly without incident.

Do you have any suggestions?