My girlfriend's mac notebook running os 10 was freezing all day today, and now when it turns on it freezes on the loading screen. Despite several reboots, it will go no further than the loading screen. Any help? I don't have a clue about macs...

Try a reboot with the key combination "Command + S" It should take you to a terminal prompt. Once it finishes loading, type in

Repeat this process twice and everything should be fine. The fsck command runs a system check on the hard drive and repairs any erros that it finds. The -fy forces a repair routine and tells it to say yes to everything. Don't fret, it cannot harm any data on your drive.