Exactly what is INSIDE the video you are editing?
Codec & Format information, with 2 links inside for you to read http://forums.adobe.com/thread/1270588
Report back with the codec details of your file, use the programs below... A screen shot works well to SHOW people what you are doing
http://forums.adobe.com/thread/592070?tstart=30 for screen shot instructions
Free programs to get file information for PC/Mac http://mediaarea.net/en/MediaInfo/Download
What computer operating system is your Photoshop Elements 12 and Premiere Elements 12 running on?
Have you updated Premiere Elements from 12 to 12.1? If not, please do so via an opened project's Help Menu/Update.
Let us try to cut to the video properties by you posting the brand/model/settings for the camera that recorded the video? Also, what is the file extension of the file involved?
Where are you only getting sound and no video display....
a. Elements Organizer 12 accessed from Photoshop Elements 12 or Premiere Elements 12
b. Premiere Elements 12 Expert or Quick workspace
Do you have the latest QuickTime installed on your computer?
Is the issue confined to
a. this one particular piece of footage
b. all footage from the camera that recorded the video
c. all footage imported into the project independent of camera/format
Keep in mind that there is a difference in a codec being installed on your computer and that codec being available to Premiere Elements.
Let us start here and then decide what next.