Even ATR won't be able to help with this one until you provide a lot more detail about how, what and with what.
Within Elements, the preview files screen.... I use to be able to see a preview of any pic or video that I had downloaded or saved in Elements. as of last week I can only see the pictures and get an error message video not supported. However when I go to edit, I can see along with full use of edit.... just makes it time consuming to load for edit each clip until I find the one I am looking for.
Please provide more detail... Computer, graphics system, version of Quicktime, operating system, camera, footage type, etc.
Please clarify where you are not seeing your video clips and/or display in Premiere Elements 11 (for now presumed on Windows 7, 8, or 8.1 64 bit).
Edit Area monitor
or Preview Window from double clicking file in Project Assets or Timeline
or both? Or are you in the Elements Organizer 11 instead? Or are you in the Expert workspace Project Assets and the imports are importing to give thumbnails without images in them. In this example, this file does have its image with its thumbnail.
When you could preview movies, what types of video clips were they? Please tell us the properties of those videos - such as video compression and file extension as well as frame size and frame rate and as much as you can tell us about these video files. Now that you are having problems with video clips or display, are you using video clips from the same source as before? If you have video clips that worked before, please try them now to see if they still work in today's situation.
From your description, I am getting the impression that you are dealing with images in Project Assets thumbnails rather than a project video display error. But, we will not know until you clarify the presentation of your issue.
Please do not hesitate to ask if you need clarification on any of the above.
Thanks for taking over ATR!