What computer operating system is your Premiere Elements 11 running on?
Is this "it worked before but not now" or "it never worked before" situation?
What are the properties of your source media and what are you or the project setting as the project preset?
(please refer to Edit Menu/Project Settings/General and the readouts for Editing Mode, Timebase, Frame Size,
and Pixel Aspect Ratio)
Where are the source media for this project stored?
Where are the Scratch Disks located and how much free space is at the locations?
Let us start here and then plan troubleshooting strategy.