Sometimes if site is down during PR update (or even not sometimes but always) it looses its PR. But after your site is up and running again it regains its old PR very fast. I was in similar situation and PR was restored in 2 months.
if your site is back online you should be see your PR back soon...exact time depends on value and PR from pages pointing to your site...the higher the PR of your backlinks...the faster you will be reindexed/ reappear in the SERPS
Same thing happened to me as well... I was changing hosting on 11th and lost all my PR a week ago. Everything else remain same. Have no clue how to get back on track.