So I thought I'd try a modest (200 MHz) overclock using Easy Tuner which seems was a bad move.

I got one boot to a windows BSOD of inaccessible boot device and now it won't even make it to the BIOS. I've tried resetting the CMOS both through the jumper and battery and swapping the memory into a different slot all with no luck. Anything else I should try before I start scraping together the pennies for replacement parts?

Does it show any signs of life when you hit the power button now?

That is odd. A 2500K should be able to overclock much more than that without any problems. And there are safety protocols that are supposed to prevent any kind of permenant damage anyway. I've never destroyed a CPU by overclocking and I've overclocked 1ghz.

Most motherboards these days will automatically go back to a non overclocked state if you overclock so much that your computer crash. You might just have been very unlucky with your CPU or your motherboard is really bad. Either way the 2500K is not exactly new so it might be best to build a new system. If you are short on cash then a used 2500K is not that expensice at this point. You can use a 3570K or 3770K too if your motherboards support it. Most Z67 motherboards got a BIOS update so you can use them with Ivy Bridge CPUs.

Did you touch the voltage at all? If it killed itself on just a 200MHz increase without changing anything else, it was probably on it's last legs anyways.

@the_devoid: It starts up spins the fans for a couple of seconds then restarts. Every now and then it gets as far as the BIOS splash screen and then does the same.

No voltage change, I was just putting a tiny boost to the multiplier to see how it would work out. I did get the 2500k near launch so it has more than done its time I guess. Everything around it has been replaced at least once bar the fans and case. I'm lucky enough to have the spare cash to build something new so a Sunday on PC Part Picker looks like my future. Thanks for the advice.

Remove the cpu and see how it starts without it. Then if everything looks normal put it back and try to start. At that point you will know if the CPU or something else is broken.

It does seem odd that such a small OC would kill it, I've had my 2500k sat at 4.4ghz for a solid 3 years now, no probs. I'd assume if it has gone if was gonna go whatever happened, and this was some slight coincidence.