I found this solution on another post:
Hey everyone I know the solution to this:
In the app store just clik the purchases tab while holding the option key, you will be able to download Mac OS X Lion again. Just re-install it and it will work after(this worked for me).
My reply:
I have the same problem on my MacBook after installing Mac OS X Lion. I just tried to follow Luis solution, but Mac OS X Lion it not appearing under my purchases when I clik on Purchases tab. BUT, when I search for Mac OS X Lion, it show up and says INSTALLED... so how do I re-install it????
Would love your help so I can talk to my family on Christmas Day in USA. Thanks! Alice.