Thank you for clarifying in your post, then you did eventually try Rufus with a successful burn, at least, according to the Rufus. Now, since this has been a Windows machine previously, this means that Secure Boot will be enabled.
I recommend going into your BIOS, and to disable secure boot, save changes, and reboot. Hopefully the USB drive will show in the list after that. Additionally, make sure that the USB drive is plugged directly into your computers USB port, don't use a USB hub.
IF you are still having issues, go back into your BIOS, and make sure that he BIOS can actually see your drive at all, and if it does, select it to boot from. If the BIOS don't see your USB drive at all, this could be a sign that your USB drive has possibly failed.
USB flash drives have limited write cycles, far more limited then SSD drives. So, if it has failed, get yourself a new USB flash drive, their cheap, and repeat the burning process.
Lastly, I also hope you double checked the SHA256 checksum hash, to insure the ISO you downloaded, that its integrity is in good shape.