Difficult to say what exactly this could be.... many variables. Best way is to eliminate as many variables as possible then proceed in a step by step process of elimination.

First of all make sure that the CPU fan is running. Many MB's have a fail safe that shuts them down if the CPU fan fails.

If you don't have a UPS you could have had a power failure that caused an improper shutdown. This can cause very strange things to happen. Shut down the PS with the main switch on the PS and then unplug the Power Cord for about 30 seconds. This will clear the memory of any corrupted data that may be retained. ATX PS's still supply voltage to the SDRAM\DDRAM even with the PS switch in the off position. If still no boot, cycle the PS switch on and off about 7 or 8 times. Some MB's, such as Abit, have a failsafe Bios that will intialise after about 7 failed boots. Scrambled video card drivers ( after a power failure ) can also cause the boot shutdown problem. A common occurance with VooDoo5's for example.

If you've got another known good PS that you can use then install that first of all to save yourself some grief.

If things still don't work then you're going to have to eliminate things one by one. Disconnect all USB devices if you have any and see if it boots. If not, then disconnect your CD-Rom, CD-RW, Sound Card, NIC Card, Modem IDE Drives and finally your IDE drives one at a time. Get it down to where you only have the CPU, Video Card and one stick of Ram installed. You might also have to have the Floppy installed, although this varies from MB to MB. Hopefully, somewhere along the line, the danged thing boots. Then you can start adding items one at a time till it fails....or not.

Could of course still be the CPU and\or MB but you can only find out by a logical system or testing and elimination. Wouldn't hurt to get a good digital volmeter as well. A $29.95 Radio Shack model should suffice.