Rebuilt PC fails to POST

Rebuilt my PC today with a custom water loop, go to try and start and it will not POST and gets stuck in a "boot loop" without even booting.

Tried:

1) resetting CMOS
2) checking wth only one stick of RAM
3) checking all Cables are seated correctly.

Seems to get stuck in this 14 seconds on, 4 seconds off loop. Fans work, pumps work, just no POST.

Where do I even start?

System: i7-4790k GTX 980 Strix, 8gb hyperX savage ddr3, 240gb hypetX savage boot drive, 500gb 850evo second drive, Gigabyte Z97N-Gaming 5 mITX motherboard, EVGA supernova G2 750w psu

Thanks!

Update:

Unplugged all front panel switches/LEFs and it now turns on. However, I now have no video output on both motherboard and GPU, using both hdmi and dp.

This was a working system before I put water blocks on it, now it's just being an expensive desk decoration

Weirdly, I had a very similar problem.

I bought the new EVGA DG-87 case and upgraded my watercooling loop. Rebuilt the same PC I've used for four years into the case and it wouldn't post.

At first I was getting blinking CPU and RAM LEDs that, in my motherboard manual, didn't mean anything unless they were staying solid, so I assumed the system was constantly rebooting before reaching POST.

After some fiddling, I started getting six beeps, something else that wasn't in my board's manual.

Stripping it down to one RAM stick helped but then it was going back to the problem behavior at random points in the rebuild, and then still being problematic when taking a step backwards.

I ran the computer outside of the case and it worked 100% fine.

I'm pretty sure I narrowed it down to being the case. I was able to replicate some of the problems I was having by plugging in the USB2 front panel to motherboard header, but I also suspected that some part of the case was shorting the board too.

I've RMA'd the case and am awaiting a replacement.

I know this isn't much help but it was a weird problem that I couldn't quite pin down. The only sure thing was that there was no issues running the computer outside of the case.

Sounds like it could be CPU overheating protection?

Reseat the heatsink on the cpu (cleaning the paste off and re-applying it).

Okay so at about 3am I finally got it working.

Turns out reading online others had an issue with front IO connectors stopping post and video out. I unplugged them all and it started.

CPU temps are good though, as are GPU. A quick blast on overwatch saw my GPU temps go from 78 degrees to 32!

Just playing around with D5 pump speeds and stuff, but I'm really impressed what whatercooling has already done.

Now, to investigate which is the offending front IO plug...

any pictures of youre loop? been tempted to water cool mine at some point but need to upgrade my GPU first as still running with the 970

Not yet, will be putting a build log in projects tomorrow most likely. Just putting on the finishing touches.

I had a similar problem last night from a duff card reader.

Reboots would freeze at post.

Unplugged it and it continued booting.

Worked fine the other week!

Sounds like a nightmare. I've been building 20 yrs. Never had an issue. Then my last pc build, had 4 faulty motherboards, then the PSU blew up! Thought I was never going to get it working. Have to wonder if component quality has gone down to save money.