I don't know what to do :'(

You'll probably have to re-download the installer if you own the game on gog. As for the store page, the game has been removed from gog because Carpe Fulgur didn't check his email. Someone really ought to contact him about this as he promised the game would return to gog almost half a year ago.

How do I redownload the installer? (I assumed I had to do something along those lines which is why I thought it would be on the store page)

Are you using Galaxy? If you are, instead try downloading the installer from the main GOG website. It will always be there. Go to your library, click on the game card, the installer is there (see pic attached). Also try a clean reinstall. Remove all the old files and reg entries from your computer (manually, if you have to), then install again and see if works.

From previous threads, I thought a repair fixed mucked up installs?

I don;t use Galaxy so I'm asking.

I couldn't repair because it wasn't installed.

But anyway, I fixed it, thanks guys :D