I have a problem with my machine not booting and not allowing me to access the BIOS. When  power on, with no further interaction I get a flashing cursor in upper left hand corner. If I press delete to access BIOS OR try to boot from DVD, I get nothing at all – just a blank screen. I have reset CMOS, unplugged peripherals, Swapped RAM, all to no avail.  I have not yet replaced the PSU, I will do that next. Next I would replace CMOS battery, even though it doesn’t show any signs of weakness.
The ONLY way I can get to my BIOS is if I physically unplug the machine, let it sit for a few minutes, and then restart. That works almost all the time – maybe 100% of the time.
I’ve gotten the impression that some logic circuit gets reset when there is no power available and the capacitors are fully drained. But I don’t understand WHERE that circuit is – is it in the MB, or in the PSU? So I’d love to hear from someone who understands the fire-up sequence better than I. How intelligent is the PSU, and what does it communicate to the MB?
Thanks!
System is 2 years old. Home-brew. Components are :
Windows 10
Motherboard Gigabyte Ga-z170x-UD5 TH
32 gigs G. Skill memory
Boot drive Samsung 512G MZ-V5p512bw
2^nd drive –960 G  PNY SSD 7CS2211-960-RB
Video Zotac ZT-7115-20l GT
DVD – HP DVD 12651
PSU - EVGA Supernova 650w (2 years old)

When I can boot (after unplugging machine and waiting for power to drain), the machine will boot just fine, regardless of if I go into the BIOS or not. And the machine runs like a top then! It's only the booting that is an issue. My next step would be to replace the motherboard. Also forgot to mention it's an i7 6700 chip, DDR 4 memory.
 

But, you see, the issue is - why does it get detected when power is drained, but not otherwise? When power is unplugged, then re-plugged, it behaves just fine.
Peter

Does the problem coincide with any hardware changes (added SSD) you made or just suddenly crop up?

Make sure your mobo BIOS is up-to-date (BIOS F4 improved M.2 device compatibility)..Your mobo has DualBIOS, see if using the other BIOS matters.

When plugged in/turned on, the PSU sends the mobo a 'keep alive' +5 VSB (Volts StandBy) that retains the BIOS settings even if the CMOS battery is dead/weak/removed and provides power to the board components that intialize the startup routine when the power button is pressed..If the PSU is turned off/unplugged and the CMOS battery is dead/weak/removed, BIOS settings will revert to default settings.

Unplugging the PSU will also reset the PSU if it has 'latched off' due to certain PSU protection mechanisms (Over Voltage Protection, Under Voltage Protection, Short Circuit Protection, Over Power Protection, Over Current Protection, Over Temperature Protection) that have been tripped, in which case the PSU won't even turn on at all.

Tough call..I'd update the BIOS if needed, replace the CMOS battery (CR2032 for $5.00), then start swapping out parts..Maybe take out the Samsung SSD boot drive and go buy/borrow an el-cheapo HDD ($15.00) for testing..Or try the PNY SSD as a boot drive..The PSU would be the last thing in the list of things that I would suspect.
 
 
EDIT/ADD
 
Now that I had the neccessary 5 cups of coffee to wake up this morning , something else (simple) you could try is to create a bootable USB flash drive with Rufus..Set the First Boot Device to be the USB flash drive (while it's into a USB port) in the BIOS and see if it consistently boots from the USB drive..It will boot into DOS at a C:\ prompt..Just push the power button to shut off your box and remove the USB drive.

That would give you a hint that something is amiss with the Samsung SSD boot drive.
 
 
 
 
 
 
 
 
 
 

Thanks for all your replies (which I didn't see notification of!). Bob16314, I had already done everything you mentioned and the BIOS was updated (and I'm pretty sure my board does NOT have a dual BIOS, though many Gigabyte boards do), new battery, etc.. It wouldn't boot consistently from the USB stick...I think it did once and then no more....same no BIOS entry when SDD boot drive removed, etc.  Polarbeardj, I did swap out the PSU for a new one...no change. What's odd is the problem seemed to be to be a progressive one, like a HD going bad, but even when I bypassed the HD it still kept failing. I think I swapped out everything except the CPU and the MB, so I decided it must be the MB. I RMA's it to Gigabyte, they tested it, found no fault, shipped it back, I reassemble....and here I am, typing this on it. No problems at all. I absolutely don't understand it. In the frenzy of swapping everything out, it's possible I did not always test exactly as I should have, not exactly lab conditions....but I'd say there a slight chance it was the PSU (I kept the new one in). Or maybe a short in the board, a stray wire or screw.....but I don't think so. Really, I've come to the conclusion that my MB just wanted to take a road trip.
I have to give a plug for Gigabyte's support here, they were really great, despite (from their point of view) me wasting their time. The turnaround was fast for something like this (maybe a week or so) and they shipped it back to me for free. I sorta expected to never see it again.
Again, thanks for your answers! And it's nice to be back "home". I was limping along on an old Dell Optiplex and it was a wee bit on the slow side, you might say.