BBB
What computer operating system is your Premiere Elements 12 running on? Did you have the chance to update 12 to 12.1 using an opened project's Help Menu/Update? If not, please do so when you can.
What are the properties (including file extensions) of those 5 clips that apparently successfully imported into the project? Did you save close reopen the project to run into the no launch situation?
What edits if any had you applied to the Timeline content before you left the project for a short time (assumed saved closed)? After the project was saved closed did you move, delete, or rename any of the files/folders that were imported into this project?
If you cannot open this first project (5 clips), can you open a new project?
Let us start by deleting the Adobe Premiere Elements Prefs file and, if that does not work, the whole 12.0 Folder in which it exists. I am assuming that you are using Windows 7, 8, or 8.1 64 bit computer.
Local Disc C
Users
Owner
AppData
Roaming
Adobe
Premiere Elements
12.0
and in the 12.0 Folder is the Adobe Premiere Elements Prefs file that you delete. If that does not work, then you delete the whole 12.0 Folder in which the Adobe Premiere Elements Prefs file exists. Important that you are working with Folder Option Show Hidden Files, Folders, and Drives enabled so that you can see the complete path cited.
Are you working with a pen and tablet device or mouse?
Let us start here, and then we will decide what next, if necessary, based on your results.
Thank you.
ATR