Try erasing the storage drive and reinstalling the original operating system using internet recovery mode. Note that this will remove all data from the internal storage drive and can only be used to install the version of macOS that shipped with the machine. For a MacBook Pro 2011 this would either be macOS 10.6 or 10.7.
To boot into internet recovery mode, power on the machine while holding Option + Command + R. Note that you will be prompted to connect to an internet connection before you are able to enter internet recovery mode.
Once you have successfully booted into internet recovery mode, you should see a screen like this:
[image|2132048]
Choose Disk Utility, select the internal hard drive (most likely labeled Macintosh HD) and erase the drive. When erasing choose Mac OS Extended (Journaled) as the format. Then close Disk Utility and go back to the screen in the above reference image. Choose Reinstall OS X and follow the instructions to reinstall the operating system on the cleanly erased internal hard drive.
Hope this helps!