Have done it before on upstairs thermostat, now it just reads all "8"'s on digital display, is it time for a new one? All wires hooked up ok and downstairs the same and fine - even after battery replacement, Thanks
I would try to remove the battery, and then shut off service to that thermostat and then return power again. That is a way to reset such things hardwired in. The battery is there for normal power outages to keep programs and such in memory. So removing the battery and turning off power could reset the microcontroller and maybe that will restore the display. If not, well time to replace... JMHO!
I had this happen recently on a commercial rental unit. By the time I got there every thing was working. According to the tenants, after replacing the battery, the display was showing all the icons. I do not know how it was corrected but it did. It really aggravates me that you need to use batteries in these high priced thermostats. They ought to have a capacitor memory.