host file cleared itself? how?

It could've been something simple like disabling a form of host protection in another program. A good way to re-block most of the dangerous sites through your host file is to use Spybot Search and Destroy's 'immunize' feature, or for added protection to use the immunize in conjuction with a program called Spyware Blaster.

If you already have those programs maybe somehow you or someone else disabled the host file protection.