Late 2009 Macbook: Won't boot up

I have a late 2009 Macbook that I had a few problems with yesterday.

It's been running fine but I opened the lid yesterday and the pinwheel appeared at the login screen. This didn't change for a long time so I powered down. Now when I power up, the screen lights up, I get the welcome sound and the Apple logo appears with a progress bar beneath it. Once it reached the end, which takes about 5 minutes, the Apple logo is replaced by a symbol that resembles what I can only describe as 'Forbidden' - circle with a diagonal line through it.

Any suggestions as to how I can overcome this, please?