Spotlight doesn't work anymore in OS X Mountain Lion
Firsts, make sure you have enough free disk space. How much is enough? Well, if you have 10% of the drive free, it should do. 

Open Terminal and execute the following commands one after another:

sudo mdutil -i off /
sudo mdutil -E /
sudo mdutil -i on /


Now Spotline should start indexing your drive. This may take hours but you should get search back after it ends.

Before or after you do that, you can check Spotlight status in Terminal using:

sudo mdutil -sv /