When you open your project, PP will ask you where it is. Tell it.
Media Offline is most often caused by the links to it, which are absolute Paths, being changed. It could be a drive letter, a file Rename, or similar.
I agree with Jim, that the best course of action is to let PrPro relink that media, when it asks "Where is file ____ ?" This article goes into more detail: http://forums.adobe.com/message/4598705#4598705
PS - if you have not Moved, or Renamed anything, I would definitely check my HDD's, to make sure that they are functioning at 100%.