Chae Song, PE doesn't actually store any files itself, so it can't have 5400 pictures in it--it just has thumbnails of the files you stored elsewhere.
Hopefully all that is wrong is that your preferences are corrupt. Try launching the program again, and hold down command +shift + option as soon as you click the icon. You should get a message asking if you want to delete preferences. You do.
If nothing happens from the dock icon, launch the application from your applications folder and try again there. Let us know what happens.