[Nick Ghizas]

I was having the same issue on a Mac, CS6. Cleaning the media cache and restarting PP worked.

See Move or Clean Media Cache:
https://help.adobe.com/en_US/premierepro/cs/using/WSF08BCDDB-FCD7-40a2-8290-8872EE725E6B.html#WSa41b87baf39dd9b0-4a7aee25125bce32690-8000