First of all, take a deep breath. Nothing has happened to your footage. Chances are, you have activated the ‘wireframe’ view in FCP, and it is preventing preview.

Notice the three tabs at the top of the canvas or viewer. The first should give a percentage, the second should look like this “lxl”, and the third should be a box. Click on the box and your first three choices should be 1) image, 2) image + wireframe, 3) wireframe. Select image or image + wireframe, and your footage should return.

Wireframes are there so that you can resize your image or create animation directly on the canvas without having to access the footage directly.

… so calm down.