Ubuntu – Black screen after rebooting following upgrade to 18.10

To upgrade from 18.04 to 18.10 I executed the following commands:
"sudo apt-get dist-upgrade" and
"sudo do-release-upgrade"

Upgrade proceeded normally until the system reached the reboot phase.
Following reboot the normal maroon dot progress screen was displayed for a few seconds. This was replaced with a black screen with the message "Main-Linux clean 574402/3567072 files, 7146011/14350592 blocks". And there it stopped with no further changes evident.

Ctrl-F1 took me to the normal console log in. I could log in and found everything was running normally. Disks were all present, active and with plenty of free space. Ctrl-F7 took me back to the black screen with the single message at the top of the screen.

I tried booting with the previous kernel but got the same result.

I have a spare 18.04 Linux on another partition. After booting into it a cursory look at my main system showed nothing untoward.

I have successfully upgraded Ubuntu for more than ten years and this is the first problem I have experienced.

Where do I go from here? How do I fix the problem?

Edit:

After booting into my spare Ubuntu 18.04 on another partition I used the Disks program to do file system checks on the main partitions. This reported that the file systems were clean. I then tried to reboot into 18.10 but again got the same problem, described above.

I have checked the SMART status of the SSD and it reports all is OK after self-test. The processor is Intel Pentium(R) CPU G4400 @ 3.30GHz × 2 and the GPU is Intel HD Graphics 510 (Skylake GT1)

Update:

After logging in at the console(Alt-Ctrl-F1) I can start the GUI by entering startx. For some reason the GUI subsystem fails to start automatically, as it did before.