I have replaced batteries with known good ones and have tried resetting the unit. Any ideas.

Will any thing work,blower or heat? If not check breakers going to the furnace or air handler. Some systems have a low voltage fuse on the blower control board that if it blows nothing will work it kills the 24v going to the thermostat.

Had the same problem with the same model#, display showed : LO BATT, also the mfg says use only Energizer or Duracell Alkaline? Why I don't know, things still seemed to work but because of display couldn't tell what temp settings.

BTW this happened after power outage (thanks dummies with trees who don't bother to get them trimmed) and the sudden surge when power came back on (thanks SCE) seemed to cause it? At least that's what Lux tech support (856-234-8803) said - burned capacitor (holds small charge) unit is Dead need to replace. He actually said a lot more...

*** but after he explained how to unmount the unit from the wall plate: push tab on bottom then swings up like garage door and I looked at the back and then remounted the unit and pop in the batteries it came back to life! So I'm lucky and need to count my blessings & be humble :)