After booting, the screen is black. The "select user" button and password are not visible

I just upgraded from 18.04.4 to 20.04.01. After booting the screen just goes black and I can only see the white mouse arrow at the lower right corner. The "user" and password areas are not visible at all, all is black.

I have done two things in order to start Ubutu: 1) Re-boot (alt + ImpSc + SUB) or just turn off by pressing the O button, and maybe will work as normal. 2) pressing enter (as to select user) and typing my password (as it where visible) and then enter. That works, but I do all on a black screen.

This is a Lenovo Ideapad L340, Ryzen 7

I tried to find an answer at "My computer boots to a black screen, what options do I have to fix it?"  but I couldn't find it there. Many answers there are very old and that makes me doubt if that is still applicable now (I'm just a user, not an expert) I clicked in "If an update or something else caused your problem" then "ATI/AMD Users" but the problem described under that point is different.

Any one knows what can I do to fix it?

Thanks!