Instead of telling them what's wrong, it is a lot better to remind them what areas should be checked. This will lead them to some basic troubleshooting skills down the road. For example, "why don't I see a picture on my T.V.?" Kids should be able to list the possible points of failure and go through and check the basics - both ends of the cables (are they plugged in?), the power cord (is it plugged in?), the source of the signal (is it on?), the source of the electricity (is the power strip on?), etc.
On day 1, students discovered how to design a closed circuit that would light a small bulb. On day 2, students opened the circuit and placed objects in the gap to test which objects would close the circuit and conduct electricity. They knew it worked when the light bulb lit up!