It happens all the time. I would never care what my sites' PR is. My only aim is to make good money via Affiliate products and PPC programs and that's it. No PR, No Alexa.
The PR of two of my sites remain static at PR2. I wish I get more traffic than a jump to PR 3 or 4. Let us see what happens in the coming days
Don't worry, it happens all the time, just concentrate more on link building and your PR will come back.