Well, if it sort of worked and now it doesn't, that could be caused by a number of things such as an intermittant short (such as a standoff in the wrong place sitting a very short distance from the board...contacting it), or it could be a failed power supply.
The first thing I'd do is unplug it and reset the BIOS. If it doesn't come up, I'd pull it from the case and put it on a non-conductive surface (I like phone books), connect the power supply, CPU, Graphics, and RAM, and try to start it (I also use a screwdriver to touch the power button pins when a motherboard is outside the case).
If you still don't see anything, I'd get out the digital multimeter and check the voltage on every line from start. If that looks bad, replace the power supply.