I recently upgraded from 18.04 to 19.04 but have been unable to boot into 19.04 since the upgrade. The boot process is failing somewhere after GRUB, I get a nice graphical prompt to unlock my encrypted drive, followed by a terminal screen of [ OK ] lines and the odd [FAILED] line. These scroll too fast to see what they are, but one is Failed to start NVIDIA Persistance Daemon. I can get to the Recovery Menu and have tried re-installing nvidia drivers and also tried adding nomodeset to the GRUB config. The network DNS is broken when I drop to the root shell from the Recovery Menu but this can be fixed temporarily by modifying the /etc/resolv.conf file. If I try failsafeX I get a window stating that "Your screen, graphics card, and input device settings could not be detected correctly. You will need to configure these yourself." If I try running the default graphical mode, I get sent back to the recovery menu. The other options to configure myself, don't seem to take effect when I press OK, and just stays on the option window. I am not sure what to do at this stage. The PC is a Dell Precision M6500 laptop with an Nvidia FX2800M (G92GLM) graphics card and was working okay on 18.04 prior to the upgrade.
Okay, after playing around in the dark for a bit, the installation now boots. I believe the fix was to apt remove any nvidia drivers. I did however leave the xserver-xorg-video-nouveau driver installed.
Hopefully this may be of use to someone else.