My late-2011 MacBook Pro won't switch on

My laptop has been working fine until 2 days ago where it will not go pass the grey screen stage after switching on. The apple logo and the loading wheel appears but it won't go beyond that.

I have ran a hardware check and everything seems to be okay. Does anyone know what to do here? Also, I haven't backed up my data for a while - is there anyway to revive my laptop without wiping out all the information?

Thanks very much in advance!