MacBook Pro running OS X lion stuck on boot screen
This happened to me only in 2 cases:


Incompatible SSD installed
Hard drive failure


There is a good chance that if you leave your MBP running for a few hours like that it will end-up displaying a files not found logo.

The best way to test if it is a hard drive failure is to replace it or to connect an external hard drive and install Mac OS on it.