My honeywell thermostat screen went blank. Is there a reset button?

change the batteries in the thermostat

I had blank screen also. I placed my screwdriver between the battery terminal and battery tab and voila the screen turned on. I removed the batteries, pulled the tab out, replaced the batteries, and the thermostat is working like a champ again.

Did you try turning off the power to the unit. Also is there a lock on the thermostat. Check manual maybe you need to hold certain buttons to unlock.

WARNING: When the thermostat is reset, all parameters (clock, configuration

and schedules) return to their default settings.

?? Open the thermostat's door.

?? Press and hold the Pgm button.

?? Insert a pointed object (e.g., paperclip) in the reset opening.

Remove it when the thermostat's screen becomes blank.
?? Wait one second and release the Pgm button.

http://customer.honeywell.com/techlit/pdf/69-0000s/69-1855.pdf for complet instructions

Check any batteries and replace if needed. Ensure that you use only AA alkaline, nonrechargeable batteries.

2 Check for oxidation buildup on battery contacts. Use electrical contact cleaner and carefully clean the battery contacts of any oxidation buildup.

3 Reset the system switch.

Read more: How to Troubleshoot Honeywell Thermostats ' eHow.com http://www.ehow.com/how_6596826_troubleshoot-honeywell-thermostats.html#ixzz17aWIl2Xt

Go to honeywell.com
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Bottom red highlighted box literature/image search
Type in thermostat model number
Click on arrow to right
Then click on the PDF files owners manual / installation instructions
If you can’t find what you want, the bottom left and bottom of web
Page, they have homeowners assistance and Home products to get the help you need

Hello Bad news something in 6400unit has shorted!I would try replacement.

It actually sounds like the batteries are going dead if the 24v from the furnace are not providing the power to the thermostat.

Check the battery. sometimes the stat can get an electrical shock, take the stat off the wall take out the batteries, if applicable, and set the stat down and walk away for at least 5 minutes then put it back, it might reset.

remove the batteries and pull the metal tabs at each end stretching out a little the reinstall the batteries