Strange RAM issue after moving PC

[spacein_vader]

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?

[kalniel]

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.

[smargh]

... 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.

[spacein_vader]

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