Can't get past POST screen

Hi everyone, I recently bought a new Asrock H610M-HVS new from Amazon. I built the computer woth a brand new 12100f, a 512gb Kingston sata ssd and a 1TB WD Purple. I also used an m.2 with a pcie adapter I got from Amazon. Everything worked fine for about an hour until I was done installing the drivers and a program asked me to reeboot, I hit yes and the computer restarted as expected. Only this time at the bottom right of the screen it says A2 and doesn't get past POST. Please help, on the internet I couldn't find much, some say it's the RAM, some say it's the power supply and some say it might be a storage device issue or even a faulty usb3 port on the case... I can already rule out the RAM since it's working just fine in another system. I'm studying electrical engineering and I tested the power supply (Enermax Marblebron 750W) with an oscilloscope and variable load and it didn't go above 50mv of ripple and everything was fine. Please help. I need to use this pc to work and I'm stuck.

A2 means "no bootable device found". If you can enter the BIOS make sure that you
have CSM enabled or disabled (based on what it was set to when you installed windows).
Your boot drive will only boot if the CSM (Compatibility support module) setting
is the same as it was when you installed the OS. Typically on a modern system you
want it set to disabled and to use EFI to set up your OS.

See if you can get the system to boot with a windows install medium (flash drive etc).
That should allow you to reinstall the OS if necessary.

It's quite possible the M.2 adapter's OP ROM isn't loading reliably yes. To test
that you can try hitting ctrl+alt+del to soft reset and see if it allows boot after
that. If it does then the option ROM on the adapter isn't loading consistently on
cold boot.

Hope this helps.