The last PR update was mid July, so it must be quite a few weeks ago!!!
If not, depending on how your Site is built, it is not the Domain that has a PR, but each single Page. If you have i.e. changed the names of the individual pages, you will also lose your PR.
If you lost it on the last PR Update in July, then it means, you lost your most important backlinks, or you got punished i.e. for double content. In this case you would normaly not lose a PR3 to zero at once. It would just go down to PR2.