What computer operating system is your Premiere Elements 12 running on? For the moment, I am assuming that your computer is Windows 7, 8, or 8.1 64 bit.
From what you wrote, can we assume that this worked before but not now?
Please give this a try. Place a video file or still on your computer desktop. Open Premiere Elements 12 to its Expert workspace.
Then resize the Premiere Elements 12 window down so that you can see the computer desktop and your video sample there.
Right click, hold the right click and drag the video from the desktop into Premiere Elements 12 Expert workspace Video Track 1. Release the mouse click.
Does the video appear in Video Track 1 and, at the same time, does Project Assets pop open displaying a thumbnail for the video?
We will be watching for your results.
Just an add on if the above does not work for you.... Again, assuming that you are working on Windows 7, 8, or 8.1 64 bit.
Delete the Adobe Premiere Elements Prefs file.
Local Disk C
and in the 12.0 Folder is the Adobe Premiere Elements Prefs file that you delete. If that does not work, then delete the whole 12.0 Folder in which the Adobe Premiere Elements Prefs file exists. Make sure that you are working with Folder Option Show Hidden Files, Folders, and Drives active so that you can see the complete path.
Add On...Also take 12 to the 12.1 Update.
thanks, I had to delete the folder to get it to work again!!
Thanks for the great news of "work again". Good job of sorting through the details.
Although problems with the same symptoms can (often do) have the different causes, your results will be helpful for others looking for answers in a situation similar to the one you encountered.