If it is plastic (acrylic, etc), sand lightly with 800 silicon carbide paper, then 1000, then 1200, then 1500. If the scratches are deep, start with the 800. If very light scratches, start with 1000.
The 800 will remove the scratches, the 1000, will remove the scratches from the 800, etc.
Finally, with a soft cloth, use some Kit ScratchOut. This is a very fine plastic polish found in automotive stores. Comes in a yellow bottle. Polishing just a few minutes will bring it back up to clear.