Notes in piano roll suddenly became invisible

Hello and thanks in advance for any assistance.

I really appreciate using Qtractor for free to make music! :D Alas I'm not great with computers, but I'll cut to the chase: I was working on a composition in the piano roll and as I was scrolling up all of a sudden the notes on the screen seemed to glitch, like they oscillated up and down for a split second, and ever since then all my notes on the piano roll are invisible! I'm absolutely certain I wasn't touching the keyboard or clicking the mouse at that instant. The thing is, the notes still remain and the music will still play, but they're like invisible. I can still see them down below, but not on the main piano roll window. I was in one track's piano roll when it happened, but the problem spread to all the tracks! It will be easier to explain if I just provide a screenshot:

https://drive.google.com/file/d/1PG2tC1nPLW1NvhT0niV82BA9pF97YYhW/view?…

(Note that the "A5 (81)" black thing is there because that's where my cursor was, but it looks like the screenshot didn't photograph the cursor.)

So yeah, where are the notes? I really suck with computers and I don't know what to do. Thanks for any help! :)