TLDR, have you ever had one of those days?
I had one yesterday, nothing wanted to work.
A cheap CPU upgrade from ebay arrived in the morning and I though I'd just pop it in... 5 minutes work right?
First of all when I unscrewed the mounting for the heatsink (a coolermaster evo something or other) the mount unscrewed from the motherboard and fell off the back rather than the heatsink unscrewing from the mount leading to me having to pull the whole board and refix the mount.
With the new CPU installed I turned it on for the first time and got random MCEs from the kernel and it wouldn't boot. At this point I though I'd gone too far in trusting ebay but I figured I'd try a new bios before shouting at the seller. That installed OK going straight from F4 to F8 but it didn't help getting the machine booting so back to the original CPU.
Which now also refused to boot, **** **** **** **** ****. I wasn't getting the MCEs but it seemed to lock up part way through the boot.
To my horror I discovered I wasn't allowed to go back to the F4 bios I knew worked and could only go back to F7 which still wouldn't let the old CPU boot the system. Bollox.
All the other unnecessary hardware now came out and it still refused to start. At this point I noticed my CPU fan wasn't turning, odd. Cable connected OK and no obviously loose or damaged wires, tried it on another fan header and still nothing, tried another fan on the same header and it worked. Somehow I'd killed the fan taking it out the case. Still the bios wasn't exactly reporting silly temperatures so this didn't look like the cause of the MCEs and while a new fan brought them down a from around 55C to 35C problems remained.
As a final throw of the dice I decided to see what happened with a clean install on a spare drive. The bios worked fine with the new CPU installed and I'd have thought I'd see some weirdness if the CPU really was a dud (crashes in the bios, spontaneous reboots, random failed POSTs etc) . The new installed worked flawlessly with the new CPU, no MCEs, so it looked like the ebay seller was off the hook and the CPU was fine. Going back to the original install the MCEs had disappeared there too but it still wouldn't boot completely behaving like the original CPU and just locking up part way through.
I wish I knew more about how UEFI works but I'm guessing there was some kind microcode patching being done by the Linux UEFI boot entry and doing a clean install with the new CPU installed fixed it.
Anyway it still wasn't booting but it appeared to be graphics driver related rather than the CPU which was confirmed by going back to the new test installation and adding the nvidia driver. Now that install behaved like the old one. Poking around the logs I noticed some references to Intel graphics and IIRC Prime. It looked like the new bios allowed the nvidia driver to works as hybrid graphics and part way through the boot process I was being switched the Intel which looked like a lockup. Disabling the Intel graphics in the bios sorted that and I finally had a booting machine with my original install.
I tried a couple of games and everything seemed lovely, until I rebooted. Lots of reboots later I discovered that it worked properly maybe 1 in every 10 reboots. The rest of the time things were a slideshow, games were at maybe 5-10 FPS and even the desktop seemed pretty sluggish. Clock speeds were the first port of call but they looked OK which left the PCIe bus. The only control my mobo gave me for that was to choose gen1, 2, 3 or auto. So I gave gen 2 a go rather than "auto".
Bingo, everything now works.
Obviously I'm relived but why can't life be simple?
Oh and I had to do some actual paid work after this and neither of those jobs really worked properly either.
EDIT for anyone interest I was upgrading an i5 4460 to an i7 4790 on a Gigabyte Z97P-D3 with a 1080 graphics card.