This is not the normal "black screen"issue and I was shocked to eventually find out why. The normal advice of reconfiguring Xorg didn't work. Even booting into "Recovery Mode" did not help.
Here is the short end of the stick that fixed it:
sudo apt-get install mdm mate-desktop-environment
Yes you got it right, mdm and the mate-desktop-environment / gnome were somehow uninstalled. This must be whe........
When things go wrong your video is basically unplayable or the first video plays fine and then freezes when moving on to the next. Generally if both videos weren't produced with the exact 100% same settings you will have issues. You can try the basic concat but it often won't work right.
Solution for me:
My example uses 3 videos in total so "n=3" and a=1 to include audio.
ffmpeg -threads 12 -i file1.mp4 -........
In my case I already copied the VIDEO_TS folder to my hard drive.
Here is the command for mencoder:
mencoder -dvd-device "myvideobackup/VIDEO_TS" dvd:// -ovc xvid -oac mp3lame -lameopts cbr:br=128 -xvidencopts pass=2:bitrate=-700000 -o myvideobackup.avi
Replace "-dvd-device myvideobackup/VIDEO_TS" with the location of your backup files or the /dev device of your DVD drive.
The -o myvideobackup.avi is of course the name o........