It would be a good time to swap the HD for another startup system. You could do that with a thumb drive startup disk that you or someone else setup already, you could use a usb cable connected to any external drive that had this... you could probably get one online if you've never made one. You could pull that drive out and connect it to another mac, then use disk utility on it. Here is an article with some helpful stuff... http://www.idownloadblog.com/2016/02/25/how-to-start-up-your-mac-in-internet-recovery-mode/

It would be a good time to swap the HD for another startup system. You could do that with a thumb drive startup disk that you or someone else setup already, and you could use a USB cable connected to any external drive that had this. You could probably get one online if you've never made one. You could pull that drive out and connect it to another Mac, then use Disk Utility on it. Here is an article with some helpful stuff.