Did you ground yourself before and during working on the computer?
This can involve purchasing an inexpensive wrist strap designed for this purpose, or being sure to touch a part of the metal case prior to performing maintenance. Plus being sure not to perform these operations on carpet, this is an absolute no-no, regardless if one has seen others do it w/out harm.
Also, it's imperative not to come in contact with the pins of the CPU with your body, static or oil from skin can damage it. Though this is rare, it happens. Usually damage to the CPU is a result of bent pins, and these doesn't have to be long for this to happen. At any rate, if there aren't bent pins, the CPU will drop into the socket with practically no effort. I take it that you did replace the thermal paste also? Clean off all of the old on both the CPU & heatsink surfaces. If the paste wasn't replaced, it's instant failure as soon as the computer is booted.
During the winter months, static risk is higher, making it all the more important to ensure you've grounded yourself as a precaution, though it should be done year round.
Finally, have you checked to make sure that all wires & cables are in place & snug? Sometimes older (or frayed) cables will break internally during disconnection, as a result of being hard to remove. If you have an extra cable to connect the PC to monitor, switch the cables.