Welcome to the forum.
First, a couple of questions:
- What OS version are you on?
- Have you updated, or changed the OS in any way?
- Have you done any work in the Registry (if on Windows)?
- What are the full specs. of those media files - more info the better?
- What is the Project Setting?
What is the information about the files?
Codec & Format information, with 2 links inside to read
-read both links in reply #1 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
Also note that Premiere Elements project files (.prel files) are not media files. So you can't import them into a project.
Media files are the .avi, .mov, .wav, .m2ts, etc., video, audio and still photos you add to your project as still photos, audio and video files.