My pc is not booting graphically because of the above mentioned error, and I'm no genius to run pc through terminal, so please help
If you are a beginner, this is just about the toughest spot you can be in. This will help.
I had this problem today after an update. I could see the XFCE desktop did try to start, but it was failing to launch the display manager (either gdm3 or lightdm failed). End result was Black Screen with that message you saw. I'll be shocked if your problem is not caused by proprietary drivers, probably nvidia.
I felt lucky to get out of this. When the Black Screen of Death happens, I was not able to get a terminal with Alt-Ctrl-F2. Old time Linux skills came in handy.
In my case, it turned out that the nvidia drivers were not signed in the expected way. I found posts about corrections proposed for kernel, but I was happy to just get rid of nvidia drivers so the computer might start. I'll run without them for a while.
Here's what I did to fix. Use power button to restart. As the system is restarting, hit the Esc key over and over. Pay attention, stop hitting Esc as the small menu appears. If you accidentally hit escape there, you'll get bumped to grub menu and that's tough to navigate. I restarted a couple of times to hit Esc just at the right moments. You arrive at a menu with 3 lines. The 2nd line is Advanced Ubuntu Options, which is just choosing kernels or recovery modes. Cursor down to revovery mode, pick that, then another menu. I choose the "root" session. I do have a root password, I hope you do too. If you log in with password, you'll be root.
List all files with nvidia. Prompt will be pound sign.
# dpkg -l | grep nvidia
and then purge those files (whatever name) one by one
# dpkg --purge ?????
It may refuse because of dependencies, but keep trying.
You probably won't have network access at that point, so apt-get installing things wont work.
Thanks to @pauljohn32, I started with this, but found there's maybe a more automated option.
Something that is possible to try:
Boot to recovery mode by holding down
left shift or
esc while booting.
* Advanced Boot options
I selected the first
... (recovery mode) listed
At this point the following menu was not rendering correctly, but by using up/down arrows I could still see the options.
It took some time to reinstall all packages and it looks as though many issues were resolved.
This fixed my
pkcs#7 signature not signed with a trusted key situation. Good luck!