Hi Bill, thanks for your post, and sorry about the trouble!  This sounds like a wire pinch, which is causing a short circuit.  Wire pinches can happen even after a kit has been functioning fine for a while-- typically what happens is two LEGO parts with a wire in between eventually push through the wire insulation and cause the wires to touch.

If I had to guess, I'll bet this is happening on the second floor.  The GBHQ kits have a warning insert that says, "Especially with the 2nd floor bedroom lamp and 2nd floor video game light, it is possible to damage the wires while installing, by wedging too tightly between plates."  My guess is that you have a pinched wire somewhere on the second floor.

Here's what I'd recommend:

1. Disconnect all floors from power.
2. Connect just the first floor and see if that works.
3. Disconnect the first floor, and connect just the third floor and see if that works.
4. Disconnect the third floor and connect just the second floor.
5. In performing these steps, you're likely to see one floor that is clearly malfunctioning. You will then need to trace the connections from the source of power out through each of the connections until you find the pinched wire.  It likely won't be visible (i.e., a cut in a wire that you can see), so you'll have to connect each section on the problem floor to power individually until you find the section that's causing the problem.
6. You might want to begin troubleshooting with sections that have wires that pass through tight spaces (i.e., wires that are wedged between plates).
7. Keep connecting sections to power until you find the connection that makes everything go out (or that makes the wire and power supply get hot).  That's your problem connection.

Let us know what you find out.  We're happy to send replacement parts for any wires and/or lights that have become damaged via pinching during installation.

Thank you again for letting us know about this issue.  We'll work with you to make sure it gets resolved!

--Rob