How do i fix my macbook pro, which won't boot up?

My macbook screen turns on, however, after loading up to 2 thirds of the bar, the apple logo becomes a circle with a cross in it. My software is el Capitan.

Prior to this incident, I had been trying to back up my photos on an external hard drive. Yesterday i had tried to back up photos on an external hard drive using the finder and i left it to sleep overnight and to complete the process. My mum had been using it last night, and it started to freeze up when I tried to switch users and it wasn't responding to my cursor (which I could move). Therefore I force shut the Mac down. By this stage I had also unplugged the hard drive (probably my mistake). When I booted it up again the situation with the circle and a cross appeared and it hasn't changed since.

I would appreciate if someone could help me fix this problem, even if it means sending it to apple.

Thank you very much.

Harry