My dotnetnuke page was working fine the other day (through localhost - it's still a work in progress) but today something strange is happening.

I can view all of the pages, including blog posts and comments, and forum posts. But if I try to reply to them, I get the text box to fill in the reply, and when I hit 'add comment' or 'update' internet explorer says it has an error on the page and does nothing. 
It says WebForm_PostBackOptions is undefined, but I was able to post/reply a few days ago.

This also happens when I try to add a new user. But the cancel button always works (aka the button right beside the one I'm trying to push).

Please help!