This is like saying “my car won’t run”. There could be a million reasons, but without more information, it is impossible to give you specific advice.
Are you getting an error message on the front end? If so, what is it?
Are your sites really down, or are they down just for you? Have you checked them through a site like http://www.isup.me?
Have you checked your server error logs?
Are you using a CMS like WordPress or Drupal?