Screen turns off after i5 2500k overclock

Hi. I'm new to overclocking and I wanted to ask some help with an issue i am having overclocking my i5. My specs are as follows:

• Intel i5 2500k
• MSI P67A-GD53 Motherboard
• 8 GB GSkill DDR3 RAM
• Gainward GTX570 Phantom
• OCZ Agility 3 SSD
• Corsair H60 liquid cooling
• Corsair 650W PSU

I did a small overclock to the following settings (I was following a guide).
Multiplier: 40 (4Ghz)
VCORE: 1.25V

This seemed to work fine, and was stable for about 30 mins on prime95 and temps were around 44 degrees C. I left the PC on overnight to download a file, and when i got back i found that while it was still on (fans spinning, etc) the screen was on standby and all the peripherals were powered down. After rebooting the same thing happened after about 10 mins.

I just reverted back to default settings, and the PC now seems stable. I ran an hour of Prime95 + ATITool and the system was stable, and temps were all fine as well.

I am wondering what might have been the cause of it. My current theories are:

• Overclock exceeded the limitations of the chip.
• The CPU was undervolted, and I should have used a higher VCORE.
• (What I am worried about) My PSU wattage is too small for this build.

I would appreciate any help.

1. Nope. Not close to a limit.
2. Maybe.
3. Psu is easily ok.

Do u have pll override voltage enabled? Did it go un to sleep?

Did you bother to up the memory or NB(north bridge) voltages?
If no then no wonder its blacking out, I would definitely up the voltage for the memory then if it still blacks out up the vcore a tiny bit more then if more stability is needed up the voltages for the NB as well.
Just do it properly in small increments at a time and make sure your ram can take the voltage being passed through them.
If you need anymore help with overclocking just give us a shout but, I think its the memory or vcore voltages or both probably.

He's got a sandybridge. Its multi overclocking so the memory or 'nb' is likely not the issue considering its at stock or less depending on profile.

Another possible cause is did you setup your ram properly?
So if say they are meant to run in dual channel configuration make sure you put them in the right ram slots.
Check your motherboard manual for reference.
I think the vcore is the issue but the blacking out makes me think memory issue so make sure your ram is setup correctly.

Try turning off any settings that put the computer is a suspend or sleep mode and see what happens. It might be hanging when coming out of sleep mode. I've seen that problem appear on other systems I've built. Since I run DC projects anyways, I always disable any sleep mode settings.

+1 Sounds like a sleep mode problem. How much voltage do you get with the vcore at stock settings and up the multiplier 4.0GHz.