My friend's guitar makes a horrible noise whenever it's plugged in, and it doesn't register any notes at all. What could we do to figure out what 's wrong with it? We don't have any extra pickups to try out, so that's out of the question.
Check the input jack, making sure there aren't any loose wires. It could definitely be something else, though that's probably the most likely suspect.
i had a similar problem with my guitar. occasionally when i plugged it in, it would hum very loudly and the notes wouldn't play through the amp. To fix it, I used different plugs, and when I plugged it in into the amp, I would adjust the plug, turning it a little bit in the guitar, to make sure it was properly connected. Hopefully that will do something, but the problem very well may be bigger, in which case, i have no idea. sorry mate
^Different plugs as in different cables? I don't think that's the problem since he's tried it with my setup and still the same results.

pixysticks: I checked it last time he was over and it was fine, but I'll give it a once over again.
i literally just fixed this like 10 minutes ago on my guitar. try taking off the plate your input jack goes into, should screw right off. check the thing cord plugs into there should be like two wires connected to the piece so be careful. these two wires are what gets the signal. I had the same problem you did and when I opened it up I found one wire was loose and the other one fell off. I simply sautered it back on to the metal piece and its been picking up notes ever since.

really stoked that I dont have to buy a new guitar. hope this helps you