MacBook Pro late 2011 cannot boot using regular tactics
First of all, there is a good chance your hard drive may be dying, in which case you could lose all your data if it is not backed up. Hopefully you already have a backup. If not, let me know and I can walk you through that. 

This assumes you have a full backup of your computer already:

What happens when you hold Command + R while turning on your Macbook Pro with the power button? If you can get into the Recovery Partition this way, the first thing to try would be repairing permissions.

You do this by first clicking "Disk Utility" then selecting Macintosh HD. Click "First Aid" and then let it run. Once that's complete, restart your computer and see if that resolves the issue. If not, let me know and we can move on to the next troubleshooting step.