Strange RAM issue after moving PC

This week I had to unplug and move my PC (full description in My Rig,) for 24hrs so we could have a new window fitted in the office. Upon putting it back it refused to POST. Initially I assumed a CMOS battery had died so I reset it and gave it a new battery, no dice. A bit of trial and error reseating bits led me to discover that it will boot fine as long as only 1 of the 2 memory DIMMs is present. This means I'm currently running 8gb of DDR4 in single channel only. Things I have tried that make no difference:

1. Turning XMP off.
2. Trying all available RAM slots/combinations of slots.
3. Using a different pair of Crucial DDR4 2666mhz RAM.
4. Reseating all components/cables.
5. Updating BIOS.

I'm at a loss. Any one slot with any one stick of any RAM works fine, any 2 and a black screen. Any ideas welcome! Worst case am I looking at a fault on the mobo?

Check for damaged pins on the CPU/socket. These seem to lead to RAM problems, and they're the kind of thing that can happen if you've got a heavy heatsink hanging off it when you move a PC. Overtightening the HSF also can cause the same thing, but I'd remove the HSF to inspect and then refasten it carefully (with new paste) and not over tightly.

... and check for physically damaged components on the memory stick.

I once had some sticks with some tiny [capacitor/resitor] close to the top edge, and my thumb pressure when installing the module caused a single ~1mm-2mm component to crack slightly - it was barely visible, but it was cracked. This introduced BIOS timing errors for that stick.

e.g.
https://cdn.shoplightspeed.com/shops...b-ddr4-266.jpg

Followed the tips here and reseating the CPU seems to have done it. Thanks all!