You need to hold the power button for 5 seconds and the machine will turn off in those situations where you're thinking the PSU isn't turning it off.
Pull the mobo out of the case and put it on something non-static like wood, cardboard, the box the mobo came in, you get the idea.
Plug in only 1 stick of RAM, the video card and the keyboard and see if it boots up. If it does, then it may have been a grounding issue inside of the case at which point you'd remount the board still only using 1 stick of RAM and video card to see if it'll still work. If it's still working after re-mounting then just plug things back in one at a time.
if it still doesn't beep or anything then remove the RAM stick and see if it gives you any beeps. If it beeps with the RAM stick pulled out, but not with the RAM stick in, then chances are the RAM is causing the issue.
Same thing applies to the video card, pull it out and see if the computer responds with any beeps.
Dont forget of course to hear the beeps you will need to plug in the 4pin PC speaker thats generally plugged in near the pins where the power and reset button and such is plugged in.
Theres beep codes for an award BIOS. Most BIOS's are award these days, buuuut if yours is different there are other makers on the left. If you dont know yours, then I guess you can ignore that part.
If after pulling out the RAM and video card you're still not getting any response then the mobo or CPU has more than likely failed or you simply have plugged in the power cords incorrectly. Make sure you've got both the 20/24 pin main plug in there as well as the auxillay square 4pin connector on the board as well as any molex connectors on the motherboard if it has it. Some of the newer boards have molex connectors on the board for more power to select components.
But since you didn't bother to put any system specs (a must when you're wanting help with a computer after all) I can't look up your board and see what it needs.