Restoring my old website

I’ve been working to change my regular html website into a wordpress website with the Optimizer theme, but I didn’t manage to get the results I wanted, so now I want to change back to my old site.
However, when I deleted all of the wordpress stuff (I think I deleted everything), it now gives a white screen with a fatal error.

If you go to this site: http://www.dispuutiuventus.nl you get a fatal error

However the old site is still somewhat accessible because when you go to:

http://www.dispuutiuventus.nl/voorwoord.html

you’ll access the old site. All the pages work on it except:

http://www.dispuutiuventus.nl/index.html (the front page)
http://www.dispuutiuventus.nl/evolutie.html

It used to be the way that when you typed in ‘www.dispuutiuventus.nl’ it would automatically link you to http://www.dispuutiuventus.nl/index.html, but that doesn’t happen anymore.

Does anyone know how I can fix this and restore my old site? You would really help me out!