we had this problem recently- the dealer seemed unaware that it an ongoing problem with several years of Prius and there is an extended warranty on the issue. It is not related to the KAAR security system, as we were told, nor does it have to do with the 12 volt battery as we have read. We had to replace the 12 v battery last fall. It is still just fine. This is the first problem we have had with the Prius we bought in 2011....and are curious if others have had such a happening..and what the dealer did about it. Apparently this is an intermittent problem and has happened to us again just after the Kaar system was removed. We did not have cold weather at the time- altho it was in the 50s overnight.