Sounds like it's time for the usual check of loose connectors. Externally, start by disconnecting all USB peripherals except the keyboard and see if it boots; sometimes a faulty USB peripheral can cause strange behavior. Then disconnect everything except the power cord, one monitor, and keyboard, and try again.
If that isn't it, the next step is to disconnect everything from the motherboard (as in, EVERYTHING, except the CPU) and let it sit for a few hours, then reconnect only a minimum bootable RAM, power supply, keyboard, and video configuration (use integrated video if available). Use the reset jumper to clear the motherboard BIOS, then try to boot. If that still doesn't do the trick then you probably have a dead motherboard.
Abacus Model 2.5 | Quad-Row FX with 256 Cherry Red Slider Beads | Applewood Frame | Water Cooling by Brita Filtration