PC won't turn on

[imported_hitch]

Here is a brand-new assembled PC. I left it in standby mode overnight, and in the morning I woke up to find it down. It doesn't react to the front-panel power button now. All it reacts to is plugging the cord into the wall outlet - it turns on, but after about 2 or 3 seconds it would click and turn off again. That's it - no reaction to power button after that, only to unplugging it from the wall and plugging it back again. What's the problem and how to fix it?