Recently I had interesting experience at one conference. My PowerPivot plugin got disabled and I had no clue how to enable the same. After while, I figured out how to enable the same. Once I got back from the event, I searched online and realize that many other people online are facing the same problem. Here is how I solved the problem.
When I started Excel it did not load PowerPivot plugin. I found in option>> Add in the plug in to be disabled. I enabled the plugin and it worked very well. Let us see that with images.
Reference: Pinal Dave (http://blog.SQLAuthority.com)