Desktop system boots to tty1

[mahesh]

Whenever I reboot my Ubuntu Desktop system, instead of booting the the graphical login, it boots to the tty1 console login.

If I then login, and use sudo systemctl start gdm3.service, I am presented with my usual graphical front end.

I tried using sudo systemctl enable gdm3.service, but this fails.

I have tried to use sudo apt install --reinstall gdm3, this seems to work, but I still boot into the tty1 prompt.

Is there a command that I can use to reset all the graphical config and binary files to a standard desktop? Or maybe someone else has experienced this, and can point me in the right direct to fix it.

I really don't want to have to rebuild this system as it has a lot of development software installed plus secret keys, repositories etc. It would be a nightmare trying to build it again from scratch.