Thanks for clarifying. In that case there are a few things you can try. A glass file will get you the original plastic shine and is really good for flat surfaces. This is the buffing compound I use, which does a good job and works for paints and is safe for plastic. You can also try nail buffing blocks.

The original plastic surface is really shiny because it's so smooth. So if you want it to look like plastic again you have to polish it after sanding with 2500 grit.