Hi !
beginner question here. I've installed nixos within virtualbox, made a few changes to configuration.nix and eventually did a nixos-rebuild switch. 
Unfortunately, the build failed and after a reboot, grub cannot seem to find the kernel at all. Whatever the configuration I select, it always fail to find a kernel :-(
1) is this normal ? the latest modification wasn't about the kernel nor grub so I was quite surprised it failed like this.
2) when a nixos-rebuild switch failed, does it automatically rollback so next reboot fallback on the previous working configuration ?