CSM issue 2019 stealth laptop

I was messing about in the uefi/bios trying to boot from an external usb. I enabled CSM thinking it would help. Within the CSM options, i disabled everything ("do not load" i think was the option) except storage, for boot options i enabled "uefi & legacy". Now when i boot its just a black screen. I tried plugging an external HDMI monitor into the thunderbolt port but nothing there either. i can press F1 for it to go to setup but i cant actually see anything, it remains a black screen. i cant get to a bootable usb as its all black screen i can let it boot to my OS, (ubuntu), which works ok, screen becomes visible when it reaches the decrypt prompt. i tried resetting nvram from ubuntu, but does not seem to have helped anything sudo modprobe nvram sudo dd if=/dev/zero of=/dev/nvram I could try blindly changing the setting - press right 4 times, press enter, etc. would need to know exactly where to go though and seems quite risky.... any other ideas?