Connected C-Wire (previously unused) to thermostat, but still no power?
It looks to me as if you have shoved the insulation of the blue wire down under the screw terminal at the furnace end. This could be keeping the internal copper conductor of the wire from even contacting the screw terminal block.

You may have done the very same thing at the thermostat end of the blue wire. Note how in the below picture the adjacent red wire shows the center conductor as a small gap proving that it has been stripped of insulation. Did you forget to even strip the insulation off the blue wire?