A customer (or rather their young son) had a blue screen with DPC Watchdog Violation error showing, and a request to scan the computer. This is normally something to do with an SSD in Windows 8, but sometimes just with an ordinary HDD. Anyway, the scan ran to 100%, but after 5 mins, it did not shut down, so the user shut the computer down manually.
From then on, power up showed the lights, spun the disk, but nothing came up on the screen, not even a bios or an intro screen. Tried plugging in a monitor, no deal, tried booting from a CD, no deal. I went to take the disk out to check it on another computer, and there were no screws on the back of the PC! So no quick easy or straight forward analysis possible.
With nothing else to try, I removed the battery, plugged in the PSU, and powered it up successfully! Powered it down, put the battery back in, powered up again, no problem.
It always amazes me how often this trick rescues laptops that are showing no signs of life.