Check behind the 12V outlet. If fuse popped originally, perhaps the positive wire is loose / disconnected at the outlet and went to ground causing the fuse to blow. But is now floating free in the air - so no voltage at the outlet.
Check the wire where it attaches to the outlet for voltage. Sometimes if the outlet is outside the
corrosion inside the unit has caused a disconnection internally.
When you pull the 12Volt outlet from its holder, check the ground connection too, as if the positive wire is good, then the ground may be disconnected - which would also now give you no voltage reading.