Hard drive partitions destroyed by FileVault
I would highly recommend using Disk Maker X to create a bootable USB disk to reinstall OS X/macOS. Here's what you'll need:


Another Mac with access to the App Store (if the one you're trying to fix doesn't function enough to complete the following steps.
A USB drive/stick with a capacity of at least 8 GB. Also be sure it has absolutely no data on it, since it will be formatted.
An internet connection in order to download Disk Maker X.


First, download macOS Sierra on whichever Mac you're using by searching for it on the App Store, but don't install it! Next, download and install Disk Maker X on whichever Mac you're using. Then, open Disk Maker X and follow the steps listed. After that, make sure the Mac you're trying to fix is off. Plug in the USB Stick, then press Option and press the power key. Select the USB disk in the window that loads, follow the setup process, and you should be good to go! Let me know if you have any questions.