If you don't already have a current backup of all data, back up before proceeding. There are ways to back up a computer that isn't fully functional. Ask if you need guidance.
Start up in Recovery mode. When the OS X Utilities screen appears, select Disk Utility.
In the Disk Utility window, select the icon of the startup volume from the list on the left. It will be nested below another disk icon. Select Unlock from the File menu. When prompted, enter the login password of a user authorized to unlock the volume, or the alternate decryption key that was generated when you activated FileVault.
Then, from the menu bar, select
File ▹ Turn Off Encryption
Enter the password again.
You can then restart as usual, if the system is working. Decryption will be completed in the background. It may take several hours, and during that time performance will be reduced.
If you can't turn off encryption in Disk Utility because the menu item is grayed out, you'll have to erase the volume and then restore the data from a backup. Select the Erase tab, and then select
Mac OS Extended (Journaled)
from the Format menu.
You can then quit to be returned to the main Recovery screen. Follow these instructions if you back up with Time Machine. If you use other backup software, follow its developer's instructions.
Don't erase the volume unless you have at least two complete, independent backups. One is not enough to be safe.
