BIOS works, Windows won't boot, what should I do?

I finished my PC yesterday and was able to use it without any problems. Yesterday I turned it off as usual. Less than 24 hours later today I came home, started my PC and only saw the MPG logo from MSI in a gray square with the design as on the packaging. I can open and use the BIOS as normal. I have already reset it twice, both as a standard reset via the BIOS itself and via a hard reset by removing the "battery" in the mainboard.

As already mentioned, the problem is that Windows does not boot. In the BIOS itself, my SSD is displayed on which I installed Windows the day before.

Here is the hardware:

AMD Ryzen 5 5600X
ASUS HD 7970 (temporary)
500GB WD Black SN750 Gaming M.2 2280 PCIe 3.0 x4 NVMe 3D-NAND TLC
MSI MPG B550 Gaming Plus, ATX, So.AM4,
16GB G.Skill Trident Z Neo DDR4-3600 DIMM CL18 Dual Kit
500 watts be quiet! Pure Power 11 Non-Modular 80+ Gold

My question now:

How do I get Windows to boot again? Do I have to use a UEFI "Bios" or change CSM "Bios" or anything else in the BIOS?

I'm on the verge of a nervous breakdown. In the course of the next week I will write important exams and also have presentations that I absolutely have to finish and that is more or less the only PC in the house. I still have an old laptop that is older and slower than most turtles, but it still works. (if relevant)

(I have both an official Windows stick here and a license if that is somehow relevant)

As always, I trust in meaningful contributions.

Boot order (hard disk with Windows at the top)

The only thing that would come up with is:

Create Windows boot stick on a working PC, maybe you can use it to repair the boot manager.
Boot Linux from CD, then you can at least have a quick look whether the hard disk is OK, whether you can access it, etc. Then that can be excluded.

Are you using the notebook's SSD? Otherwise restart Windows.

No the above SSD. This was (of course) delivered new by Mindfactory less than 5 days ago.

To me it sounds like your BIOS has reset (before you tried it). I suspect, therefore, the BIOS boot sequence has been changed and the boot manager of the hard drive with Windows is no longer addressed. Do you get any error message that no operating system is available?

Important: Windows 10 normally needs the Secure Boot to be Enabled. If this is set to Disabled that could be the reason.

Otherwise, what if you manually select the hard drive via the boot menu (on many boards it is either F9 or F12 at startup). Does Windows boot then? If so, it is a problem with the boot order or, as you have correctly guessed, that you are in the wrong mode (with CSM or UEFI BIOS).

I just happened to discover a function in the BIOS that checks whether & how well the SSD works. It doesn't look like it is due to the SSD, according to the BIOS everything is fine with it, and it is hardly 4 days old and I'm not a PC professional but I can install M.2 SSDs.

If an SSD gives up the ghost, then without premonition.

Saving energy?

Economy settings like "Standby after a certain time" and so on.

I'll see for a moment

I have now changed the order from UEFI Hard Disk: Windows Boot Manager (name of my SSD in brackets) to Hard Disk: (name of my SSD in brackets), then I came to the Insert Boot Device menu (I can't describe it differently because I have perform a hard reset at the moment.) There you have to insert your boot device in my case the Windows stick and press any key. I did it - plugged it into the mainboard of course - but nothing happened for minutes. I switched off the PC after about 5 minutes and when I restarted I had a black screen because my graphics card was no longer recognized (the VGA light on the mainboard was on). Now I've done a hard reset and I'm back at the beginning…

I don't get an error message at all, only the MPG lettering + design from MSI after closing the BIOS and then nothing happens, the screen remains unchanged.

I'll try your 2 suggested solutions tomorrow after formatting the SSD.

If so, I don't think you need to format the SSD. This means that your PC does not complete the post-boot process, so it does not even have to start the operating system from the hard drive. Whatever the reason, now we have to narrow it down. But some component prevents the BIOS post boot (RAM, CPU, insufficient power supply, etc.)

Unplug everything… Really everything. Only CPU + board + RAM + graphics card + power. No hard drives, no running cases, no further PCIe graphics cards, no USB devices. Just the bare system. And then see if he is at least bidding to the point that he complains about an operating system.

Then test only with one RAM bar each time to rule out that one of them is defective.

Do you have a system speaker connected? In some cases it would give you an audio error message (BIOS Beep Error Codes) if your BIOS wanted to tell you something (like RAM defect, etc.).

Are you sure that the SSD that is important for booting is on the SSD
EFI partition is located?

If not there, reinstall Windows. - Prior formatting is not necessary.

With the help of the installation disk, delete all partitions on the SSD.
They are then merged into an "Unallocated Space". >
Mark this "Unallocated space"> Click "Next"…

This data carrier is then repartitioned and formatted in one go and Win10 is installed on it.

Make sure the PC is connected to the Internet via LAN during the installation.

I forgot: For the installation go to the boot menu with the help of the F11 key when the PC is started. (Page 31 of the mainboard manual)