Try a recovery disk if you have one. Alternatively, you can make one yourself. Personally, I use a program called Hiren's Boot CD to do diagnostics when anyone is having trouble booting. You can actually make it work on a thumbdrive. I would show you a link but ifixit bots are not letting me for the time being. I am not all that familiar with Macs, but there should be a system diagnostic called S.M.A.R.T. that you can utilize to determine if your hard drive is going bad. I will tell you, though, that more than likely your hard drive is failing after ten years.