Contribute Pages Not Working

Using CiviCRM 4.4.1 with Wordpress. Everything is up to date, no modifications to core code on either.

I have a contribution page that worked properly when I created it yesterday afternoon, but starting yesterday evening, it stopped working. See here: http://nytsl.org/nytsl/membership/

I can put in my contribution information, go to the review page, but when I click 'Continue' to go to the confirmation page, I get my website header and nothing else on the page, with the url: http://nytsl.org/nytsl/membership/?page=CiviCRM&q=civicrm/contribute/transact

I've seen previous discussions on this issue, but no real solutions that apply to me. Please help, since this is one of the core features for which we decided to start using CiviCRM, and it doesn't work consistently!