Why had my rendering stopped working? It was working fine until I got into making the plane look glossy, then it just stopped showing any changes as I made them. I tried working on the shape and made it into glass and it morphed a little in the rendering but does not look like its supposed too. I am using a Mac computer OSX 10.6.8. I just got this program tonight and am studying the lessons trying to learn it!
It looks like your plane has multiple materials on it but out of sight :

Your changes on this material are not visible because it's not assigned to your object (plane).
Remove the other materials or affect this one to your geometry :
1. Select your plane, Tab to go in edit mode, then A to select All
2. Select the checker material from the material list
3. Click the Assign button