It`s possible that when you get the symptom you are describing PlasmaShadow.
That due to the Cmos battery backup coin cell CR 2023 being flat or low on power the system restarted because the currently stored bios settings were lost.
If you reset a bios even when you are in windows it will force the system to restart or it may shut down completely powering off due to an interruption in power because of a bios backup battery running low on the 3.3v it is supposed to supply to retain any changes you made to the bios manually from it`s factory settings.
It would explain why the system still boots or posts, but you cannot enter the bios of the motherboard when pressing the designated or assigned key on your keyboard to do so Plasma.
Power the system off.
If you look at your motherboard you may see the old type of connection for what we call PS2 type keyboards that is round in shape, it should be purple in colour or a half colour of purple and green if it exists.
Connect your USB keyboard to the Usb ports either just above it or the first set you come to closest to the Ps2 type connector for usb connection of the keyboard.
First try the system with the monitor lead still connected to the Pci-e video card if placed in your system, see if pressing the bios key on the keyboard results in you entering the bios set up menu of the motherboard then.
If you get a black screen still, and it fails to enter the bios set up menu.
Try the method bellow instead.
If the motherboard has an on board video solution swap the video cable from the Pci-e based card.
To the video output ports on the motherboard.
see if the bios output screen image is displayed on that connected video port of the motherboard.
If it does, check the time and the date plus the year that the bios should state once in the menu system of it.
If the time and date, plus the year are wrong it confirms that the bios has defaulted back to factory restored settings.
So you will need to replace the Cmos coin cell battery : CR 2032 for a new one.
The setting for the first boot device will of changed if the board factory defaulted.
So make sure the drive with the windows os is set to number one in the boot order listing and selection.
Check the type of mode set for the Sata ports of the motherboard.
Where they should be set to AHCI mode.
Also the default graphics card to initialize first for display output will be set to on board graphics of the motherboard other than Pci-e or peg mode that should be set if you have a Pci-e graphics card in the slot of the motherboard.
And last of all look for a option in the bios for enable detection of legacy usb devices.
Set it to enabled.
Save the changes to the bios before you exit it.
Then swap the video cable from the motherboard back to the Pci-e based graphics card video output ports.
If you get a display, and windows boots problem fixed for the moment.
NB: If it turns out that the bios backup battery coin cell on the motherboard: CR 2023 is very low on power or is depleted, you need to buy a new one to place in your motherboard to retain any bios settings or changes you make to the bios.
Or the bios will factory default every single time when the system is completely powered off for some time.
The battery only costs about £1.00 to 50p from most places to replace.
And should replace it quick as you can.
Or you will have to go through the procedure described above each time you power your system on after being off for some time PlasmaShadow ok.
Sorry if the post was very long, but I had to explain step, by step what to do if the cause is a flat cmos coin cell battery on your motherboard.
Anyway hope it helps, and gets you out of the problem you are having.
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