I'm replacing my old Honeywell thermostat with a new Wifi thermostat. My old/current one is battery powered and doesn't use the C wire. I found the C wire behind the plate. I connected the c wire to the new thermostat but no power. I checked that the C wire is connected on the furnace end (which it is). What am I missing? Is there a safety shut off switch or something?
I'm guessing you are still missing a wire.
A "C" wire does not supply power it is used as a return wire (a grounded or neutral conductor). You will also need a power source conductor, it's usually red. So your thermostat should have at least five conductors labeled R-power, C-common, G-fan, Y-cool, W-heat and if you have a heat pump O-reversing valve. Also there could be more for two stage heat and cooling, and there are some other optional terminals.
But lets stay with the basic five. Make sure you have at least two conductors for power and three/four conductors for control. If you do not have enough conductors for power then you have to supply power with a battery which I am assuming that was the reason original Honeywell was on battery, or you need to pull a new cable with the adequate number of conductors.
Hope this helps.