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What is model on the unit?
Did you turn any power off to unit to replace filter and is filter inside unit?
OK only panel you had to remove was in front of coil and no door switch yes?
Lets take a quick look and make sure fuse is good on control board
Let me ask this.Was everything working and you just replaced filter and it quit?
What thermostat do you have? Is it saying no power to RC?
For some reason we are not getting power from air handler to thermostat.Is there a float switch in main drian with 2 wires connected that may have gotten bumped or control wiring bumped that would have lost connection?
With filter in bottom of unit, there is nothing replacing filter should have done.Lets turn power off to air handler and back on and see if thermostat will reset.
OK hang on, I see wires to your filter now.
Do you have a volt meter?
Lets look at that fuse.You will have to remove top panel to see it
No this is a small 5 amp auto type fuse in low voltage circuit http://i.imgur.com/kYe4Epz.png
Plugs onto furnace board
If it is fuse, it can just be unplugged and a new one plugged in
Or local auto parts
I understand. For some reason we have lost the low voltage to the thermostat.
Most likely not. If we find indeed it is a fuse, the fuse is less than a buck, If we find it to be a transformer or something, probably 30 bucks or so plus labor.
Have him pull fuse and see if it is burned ito
Looks burned to me
Did you locate fuse as shown in my pictures and verify it is blown?