UEFI completely messed up


Hello, here I am. trying to use the newly created boot-CD from the iso file. I followed the recommendation to reconfigure the UEFI settings to start from a USB Disk player - resulting in chaos... Presently, after trying everything under the sky, I can only start by using the UEFI welcome screen, then the booting screen and chosing manually one of my 6 (!) HDs. The main 2TB SDD disk doesn't start any more - that's why I started this whole procedure. Where -FGS - is the USB starting disk to be found in UEFI as the description how to set it just doesn't fit.
Help is highly appreciated.