Well, PR is only one of many factors (200+ Google tells us) that affect ranking. And the PR that Google shows on the toolbar is not the up-to-date and accurate PR that is used on the back end, which is calculated to many decimal places.
Is your PR0 merely a gray bar, or a white bar? A gray bar doesn't mean PR0, it means "no data available". That's why I ask. Google often hides the toolbar number for various pages on a site. Sometimes, but not always, it's for suspicion of selling links.
At any rate, real PR changes actually happen every day - but the toolbar is only updated once every few months. So any ranking changes that are affected by real PR already happened long before the toolbar shows a change.
Toolbar data is famously unreliable as anomalies crop up all the time. You should look into it when you see something unusual, but as long as rankings and traffic aren't adversely affected you shouldn't worry about it too much.
I get this happening across sites all the time, and they always keep their rankings and traffic, and nine times outa ten, the PR come back with the next update. Just had one come back as a PR5; previously through three updates it was PR3, PR0 and PR0. (Heh, I figured it was because I wrote a page that was highly critical of Google) As long as you're still ranking and getting traffic, I wouldn't worry.