H55M-S2H: Boot fails. Repeated short beeps, then powers off.

[starjet]

I've been using my PC with H55M-S2H mobo on it basically everyday since I built it back in 2010. And just recently, for the first time, I didn't use it for three days.
And now suddenly, it refuses to boot. As mentioned in the subject, I get a bip-bip-bip-bip-bip series of short beeps and it shuts itself off, when I try to boot it. According to the user manual, short continuous beeps are indicating a "power error". However, what kind of error would this mean?
Any tips on how I should proceed with diagnosing where the power error is? Thanks.

[absic]

This kind of problem can be really frustrating, especially when everything was working fine and there is no apparent explanation to the cause of the issue.

Best advice I can offer here is to remove all external USB devices, including mouse and keyboard as a starting point as I have known a faulty mouse to cause this kind of situation.

Also, don't dismiss the PSU as being faulty as, once again, I have seen a very similar thing happen on a PC I built for a friend. It would work fine for days and then suddenly refuse to start, only to work perfectly when he brought it to me to check out! 

If still no luck, could you post back with a full list of your system components?