Ubuntu thumbnails appear, then a black screen appears with a blinking cursor
I assume that you are doing everything right, it just does not start the x-server .
to check during the “flashing cursor on a black background” switch to the first / second virtual terminal ( alt + ctrl + f1 / alt + ctrl + f2 ) - if you see a picture like this:

so it is. then you need to figure out what exactly the "dislike" of the x-server is - analyze the contents of the log file /var/log/Xorg.0.log .
Of course, it’s quite difficult to do with very basic knowledge, so as “what if it happens”, I would recommend to start by trying to boot the system with the nomodeset parameter .