PC not booting in case

[seb500]

Hey everyone. A few weeks back my computer stopped booting up, and switching around components (Re seating memory/CPU, checking for bent pins) didn't resolve the problem. I then took it out of the case and it booted up fine, put it back and it didn't (did this a few times, being careful with how I rebuild it each time). I guess the motherboard is shorting out on the case, but I'm not sure what would cause it to suddenly start doing that.

Does anyone have any advice going forward? How to diagnose etc?

specs (may help)

i7-6800k
asus strix x99
16gb kingston ddr4
960 strix 4gb
antec 750w psu
case: fractal design define s

 

[seb500]

Over tightened heatsink? Have heard on some types of socket a slightly overtightened heatsink/retention plate can cause no boot symptoms? when it was out of the case could be due to being flat it would boot ok?

I don't think I overtightened the heatsink but nevertheless something to bear in mind. Being flat is a good shout but prior to the issue it was upright as normal?

Edit: I've added the specs to the post if it helps

 

[seb500]

may sound daft but when its in the case are you using the power button on the case? i
does the board have a power button? have you tried leaving all the headers unplugged from the board while its in the case and either bridging the power pins or using the button on the board?

Going to give this ago later, thanks for the advice !