I used acetone on the back of my monitor to get rid of old stickers from LED strips. However it severely damaged my monitor. Anyway to undo this or repair it?
Acetone is quite good at dissolving polymers. Do not use acetone on plastic before checking out whether that plastic can withstand acetone.
Let it dry for a couple hours. Try turning it on again. If it does turn on, good. If not, you're pretty much screwed.
However from what I can see from the picture you didn't really damage anything vital. You can try to polish the rough surface again, but I doubt you would get any good results.
If it does turn on, I would cover up the ugly parts with black tape or something and be happy I don't have to buy a new screen.
To understand what to do you have to understand what the plastic is and what has happened. typically cases for electronics are a fiberglass-ABS composite resin. This is a cheap mixture that has a high mechanical resilience to fracture and reduces combustibility.
When the plastic is cast at the factory it is forced into a mold and rapidly cooled. This process causes the polymer molecules to retain a high degree of internal stress. The part however is smooth because of the surface of the mold. When you use acetone on ABS, you soften is and allow these polymer molecules to partially relax the stress resulting in a rough surface as the residual stress pulls molecules together creating a rough surface (this is a bit simplified). Also you have dissolved and wiped away the dyes that make the plastic black at the surface.
To restore the finish you can attempt to mechanically buff the surface to make it smooth again but the underlying fiber glass may be an issue. Using nail polish may be another way but this is more to make it look less bad than good again.
I was taking stickers off a casetter player and I thought nail polish remover would do a better job than alcohol in taking off the residue (stupid I know now). After like 3 coats of headlight restorer the plastic went back to its clear state and i’m so happy that it worked out. I have no idea how it would do on other plastic or what residue headlight restorer would leave but I used it on clear plastic. It’s worth a try though.