What do you do if you accidently rewinded your film

I was in the process of using up a roll of film on my nikon f-801s and i accidently rewinded the film when i had only taken 18 pictures and now it says empty where the 18 used to be… If i take pictures using this roll of film that is still inside of the camera, and has not been exposed to light, will the new pictures go over the old ones? Or will they just be over exposed? Or is it fine for me to just retake new pictures using this same old film?

It will ruin the other pics. Just get a new roll and take that one out.

1) if the film is still sticking out a little, reinsert the leader of film in the take up reel and close the cameras back.
2) with the shutter speed set to 1/1000th second and f/16, cover the lens with the lens cap and while indoors, press the shutter release and advance the film until you get to number 20. This will assure that you do not double expose the last frame you shot.

If the film is all the way inside the film cassette, buy a film leader extractor and then follow the instructions in 2)