No display after in-place upgrading from 17.3 to 18.0 (mintupgrade)

[JPQ]

Hi everyone.

I recently realised that I was running a Mint version that is no longer supported (17.3), so I attempted to upgrade following the instructions here: https://community.linuxmint.com/tutorial/view/2316

After it was finished, I booted into Mint 18 and there was no desktop environment, or login prompt, only a blank screen with a blinking underscore on the top left corner. Using Ctrl + Alt + F1 I can now log into my user account, but typing "sudo mdm" outputs only a 4-digit number (which seems to be different each time I reboot).

I would be extremely grateful if someone would help me fix this. Please keep in mind that I am not very experienced using the command line, so it would be best if you included all commands I need to type in order to try your suggestions.

Thanks in advance!