I have a game installed and my computer thinks its uninstalled?

I am trying to uninstall and reinstall my pc game. It wont let me uninstall it cause it thinks its already uninstalled. So now im freaking out and i cant seem to fix it. Is there another way cause i am afraid to delete the files and empty the recycling bin. Because if i delete the stuff i think i have ruined everything! I am so high in my game and i need to install a megapack and i am so ******* mad ill put a hole in the wall. But I wont :D