Localhost not working

Whenever I go to http://localhost/~david/, it shows the list of directories in my ~/Sites/ folder, however, whenever I try to open a file (specifically, html), I get a message saying
"This webpage is not available.

The webpage at http://127.0.0.1/~david/ might be temporarily down or it may have moved permanently to a new web address."

Why is this? I have made sure that websharing is enabled in System Preferences. I'm completely baffled, since it was working 6 or so months ago when I last used it. Thank you.