@MrDreamers: Welcome to the Win-RAID Forum!
As long as you haven't created a new EFI boot partion (named "Windows Boot Manager" within the BIOS) and changed the partition scheme from MBR (Master Boot Record) to GPT (GUIMode Partition Table) you cannot boot into your previously used Windows OS after having changed the boot mode from "Legacy" to "UEFI" and disabled the Compatibility Support Module (CSM) within the BIOS.
The best way is to do a clean install of Win10 in UEFI mode onto the target disk drive by using an USB Flash Drive, which has been prepared by the tool named Rufus.
A mainboard BIOS modification is only required, if you want to use an NVMe SSD as target drive and your currently used BIOS doesn't contain the required EFI NVMe module (look >here<.
Good luck!
Dieter (alias Fernando)

@MrDreamers: Since I don't know what you have done, I cannot help you. Why don't you ask the maker of the youtube video whose procedure you had followed?

@MrDreamers: I suspect, that your issue is caused by a missing GopDriver (= EFI BIOS module) for your graphics adapter within the BIOS of your mainboard (if you use a CPU-integrated graphics adapter) or of your graphics add-on card.
To be able to boot in UEFI mode and see a picture on your monitor you will have to insert a matching GopDriver into the related BIOS

@MrDreamers: If your PC has a discrete graphics card and a CPU integrated VGA adapter, you should indeed connect the monitor to the other graphics adapter and look what happens.