Load bar at start up, then shut down. HELP NEEDED URGENTLY!!!

I've had my MacBook pro for almost two years now (it's the late 2011 model) and since this morning, the progress bar has been coming up, as well as the spinny wheel thing, on the start up grey screen.

[img]http://asset1.cbsistatic.com/cnwk.1d/i/bto/20091030/MacFirmware.png[/img]

When it gets to the end of the load bar, it just powers off the laptop, and as I've already said, this has only happened today. I've been into Apple and booked an appointment at the Genius Bar, but this isn't until next Friday and I need my Mac ASAP.

Has anyone else had this problem? If so, is it easy to resolve by yourself, or will you have to pay Apple to do it for you, and if it's the latter, how long will it take?

Thamls in advance. ~ Tom