I have a noma programmable thermostat 52-2558-8. It works this morning, but now, it stops working. It cannot start up the heating. The temp I set is 19, now it is 18 and the heat is flash, but the it does not start up the heating system. Thanks

Go to the furnace or whatever, and check for 24 volts between the W terminal and common. If no voltage check the R and common. If still no voltage, you may have a blown fuse or bad control transformer. If you have power to the R, but not the W, the trouble is in the thermostat or wiring. If you have power to the W, the problem is with the furnace. Post back with more information.

Try to turn of the power to the furnace and back on again. Tell me if the furnace tries to start. By the way, what kind of furnace? What's the age? What's the efficiency? Gas, LP or electric?

I have a noma programmable thermostat 52-2558-8. It works this morning, but now, it stops working. it cannot start up the heating. the temp I set is 19, now it is 18 and the heat is flash, but the it does not start up the heating system. Thanks
You may also try to move the System slide switch from HEAT back to OFF and then HEAT again to see if it works. Open the unit and check if any wire (especially R and W) is loose.