Horn started blaring apparently spontaneously, now won't start.

I have a 1999 SC2. Last night its horn started blaring steadily (not beeping, like the security system does) with no apparent provocation. The key fob was next to my pillow, so it's conceivable that I accidentally pressed one of its buttons, but I'm pretty sure I didn't. I went out there and tried to get it to stop assuming that I had pressed the panic button by pressing the panic button again and hitting the unlock button. Nothing happened. Then, I tried my key in the manual lock, and the security system started up its typical beeping. That's the really weird thing, because for awhile it sounded like the car had two horns, but I think it was really just modulating between two intensities: high when the security system was beeping, and low for the rest of the time. I popped the hood to look for rats or mice chewing on wires, but didn't see anything (they could have been hiding, of course). Eventually, the horn started "flickering" apparently randomly between the original medium blaring intensity and a lower intensity, and eventually died out completely. Since then, the car has not been responding to the key fob at all, and the engine will not start (or even try to turn over). However, the stereo still works, and the gauges light up, so it seems like this isn't a battery problem. Even weirder, when I close the car door and sit inside of it, the dome lamp flicks on and off at about the same frequency the hazard lights used to blink when the dome lamp is set to the "door" position. Even when I turn the dome lamp off, I still here the clicking sound that the hazard blinkers make, but they aren't actually blinking outside. Maybe I just totally don't understand how this security system works, but I'm afraid that something big like the onboard computer is fried! Anybody else seen these symptoms?

You didn't give us any information like mileage or time of ownership. Personally speaking I would say that you need to pull the battery out and have it tested at a parts store and go from there. If it got cold and the battery is weak as it gets colder then the battery will lose charge. If you have a DVM(digital volt meter) check the voltage on the battery then check for resistance and draw on the battery. But I still think its just an old battery, but I have been wrong before.

I went to Saturn for a 30 point check, and they said that the keyfob had shorted out, causing the security system to continually arm, and that the horn then burned out its own fuse. The car did start on its own later, so the security system does seem to be the culprit there. That explanation doesn't seem to cover everything, but hopefully it'll just work for awhile.

The RKE module is behind the back seat in the passenger rear pillar. Not under the hood.

Ive never heard of one "shorting out" before. But if it did happen then i would assume the RKE would set the Passlock as default measure. This would explain the horn and lack of the engine turning over.

You can get another RKE unit from thejunk yard and have it programmed to your PCM. Not very hard at all.