Help! iMac won't load past apple screen!

I just updated my software via Apple Software Update about half an hour ago. And when it restarted, it wouldn't load past the apple screen (white background with grey apple and loader at the bottom).

So I tried turning it off and giving it a few minutes to cool off, but it still won't load!

I've never had a problem with my iMac before. And I don't have the discs or anything because this iMac is the Mid 2011 one. And I never backed up with Time Machine.

What's wrong with my Mac and what do I have to do to get it working again?

Thanks in advance.

I have a wireless keyboard and trackpad. Will I be able to follow your steps even though my input devices aren't connected with a cable?