You said you're computer has been crunching practically 24/7. At temperatures as high as 70*C, it is very possible that components in your CPU started to fail. When memory fails, a lot of motherboards will beep at you. My motherboard came with header pins for LED diagnostics, maybe that will tell you where the system is failing to start. If it doesn't come up with anything your motherboard could be fried.
How big is your power supply, because stressing a computer 100% for a long time can cause your PSU to over heat and if you didn't give yourself at least 25% headroom on the PSU's wattage rating it is very possible that your PSU was struggling to keep your system running and all it takes is a little over-voltage because of lack of current to fry components on your motherboard.
I know someone who had a PSU that was just large enough and it fried his motherboard and video card. I would also get a Kill-a-watt to see how much power your system is drawing once you get it repaired. Keep in mind that if you have a 650-watt PSU, it would be very unwise to let your computer draw any more than 500-watts under full load with at least an 80 PLUS Gold rating.
A little bit of information about what you're running will help us gauge how much power draw your system had (at least at stock,) and we can take some guesses about what components could have failed.
Edit: I just read that you got it working, but here is my 6-sense: Personally when I overclock my rig, I draw about 450-watts and I have a 800-watt PSU to power it. Keep in mind that your PSU powers your entire system and if your PSU is cheap, it will take other things down with it. I have an upgrade on the way for my system, and I'm looking at running crossfire 6870s and I'm investing in a bigger 1000-watt PSU with an 80 PLUS Platinum rating. When you're over-clocking you never want to take chances that it could struggle to provide enough power. Too much is always better than too little.
Edit 2: Also keep in mind that the hotter your PSU runs the less efficient it becomes and without adequate air flow the issue can "run away," until it can't handle it anymore.