The assembly lines had been shut down following the Mar. 11 Japan earthquake, and resulting tsunami that is still causing trouble for the industry.
A Toyota spokesperson was not able to confirm restored production from one of three battery factories damaged during the natural calamity, and which normally supplies the Prius assembly line.
It is speculated by those in the know that the battery plant could still be out of business. If so, this would fit with previous Toyota warnings that U.S. Prius availability will be limited due to the affected battery plant.
[Source: Autoblog Green]