I cannot quite determine what screen that is. Can you provide any insight?
Those squares with the numbers in them are actually placeholders or ASCII representations of the font.There's nothing useful on that sceen display, at all. Going back to Condobloke's reply, we need very specific specs on your hardware.
FeaturesG'day Helen and Welcome to linux.org
Give us some specs......make , model, ram, cpu etc etc
Tell us the story....what was on the pc prior to your upgrade attempt
which instructions did you follow to upgrade
Do you have important data/music/pics on that pc ?....do you have a backup?
I had the exact same problem when I upgraded LM 19.3 to LM 20.
For me, I think something went wrong 'during the upgrade' as I followed those instructions well.The display from the OP's screenshot is useful, in the same way that dumping a binary file is useful
Linux Mint has some information on the upgrade path to Mint 20. I'm not saying that those instructions were not followed, but the result indicates a problem took place either in preparation for the upgrade or during the upgrade. When in doubt, go to the source.