My MacBook Pro late 2011 turns on but will not move passed the apple logo and the waiting display icon, what can i do to resolve this issue.

I decided to restart my laptop due to the fact that it was running unusually slow, when I did so it did not go to the apple power screen at all it just shuttled off. I then proceeded into pressing the power button when I did this it did do the chime but it stayed on the apple power on screen then a updating bar was under the apple logo for about half a minute and then it just stayed on that power screen with a loading sign on it, I waited to see if it would go into the log in screen for hours it never went passed that screen! What is wrong with my MacBook, what can I do to fix it?