Ouch!
Yeah, dust is a major evil. Any component can fail given time, but a layer of dust on the components restricts heat dissipation, causing them to age more rapidly. If what you heard was more of a *snap* than a *click*, you've probably blown a chip or capacitor. Hopefully it was only in the power supply, and not on the motherboard.
Under no circumstances should you try to turn the box on again! Open the computer and inspect
everything carefully, checking for burnt or cracked components on the motherboard. Also, as strange as it sounds, use your nose. Even if a blown chip doesn't show any outward signs of damage, it can have that very distinctive (and foul) smell of smoked silicon, which you should be able to home in on quite quickly. Sniff the power supply; if you get the strongest smell from it, it's toast. If you have access to a voltmeter, you may be able to test the supply by itself, depending on whether or not your box has a "hard" on/off switch wired straight to the supply. If it does:
- Disconnect
all power leads to the motherboard and peripherals.
- plug in and turn on the supply.
- Set the voltmeter to measure DC volts, and select a voltage range of >12V.
- Connect the black (negative) lead of the meter to one of the black leads coming out of the supply's wiring harness.
- Connect the red (positive lead) to other wires (red, yellow, etc.).
If the supply is working, you should, at the very least, be able to measure +5 and +12 Volts somewhere. If not, she's toast; replace it.