After performing a fresh installation of Windows & Ubuntu on my PC, my motherboard stopped detecting the NVMe SSD to boot from.
This stemmed from the fact that I installed the latest chipset, WiFi/BT & Audio drivers from Gigabyte for Win 10, but didn't update the BIOS(in retrospect).
Clearing CMOS by shorting the 2 pins at the bottom-right of the motherboard didn't help, neither did loading optimized defaults for the BIOS. I just chanced to think that maybe updating the BIOS could help & poof! That did help with the issue. So pack an empty FAT/32 drive with the BIOS zip extracted & go into Q-Flash mode from the BIOS loader screen in order to flash it onto your motherboard.
Hope this helps!
Akash