umm I've run Prime95 for 10 minutes with no errors, then had errors appear much later. I used to do 20 minute tests, but would find games would crash after about 4 hours of play. Another prime95 run showed an error after 1 hour of success.
If CPU thermal throttling was on, it shouldn't be a heat issue though. If it got too hot it'd throttle back, if it got to the damage point, the CPU would have shutdown. If the heatsink was so bad it couldn't control itself fast enough, it wouldn't have finished the boot sequence. The OP would've needed to disable this on purpose, and would require a slapping. (Edit: keyword "shouldn't" this could still be the case as El_Captain has pointed out)
We also don't know other voltage settings. It is possible something has been over-volted too far (which is why we never recommend auto overclocks).
However despite all the CPU talk, I'm still thinking its because you bought an Asrock, I had the same experience with a P45 Asrock, mine still booted however. Changed to a Gigabyte and was amazed at the difference of its ability to overclock at lower voltages and then not die.
Anyway link to the same thing. 3.6Ghz i7, left overnight, dead Asrock next morning:
http://www.tomshardware.com/forum/ [...] ost-needed
Motherboard got RMA'd.
A quick search on google shows a suprising number of 'My Asrock X58 wont boot' posts.