I just tried to boot up my MacBook pro running osx 10.7.3 and all I get is the screen with the apple logo and the spinnIng thing ( not the beachball of death)
The following support article is somewhat outdated. Where it refers to booting from an installation disc, you should boot from your recovery partition (if running 10.7 or later) by holding down the key combination command-R at the chime.
Mac OS X: Gray screen appears during startup
Retrieving data ...