Launch Disk Utility, then select the icon of the FileVault startup volume ('Macintosh HD,' unless you gave it a different name.) It will be nested below another icon with the same name.

Take this step only if you use FileVault 2. You won't need the backup unless something goes wrong-which is always possible. If you don't already have a current backup of all your data, you must back up before taking any of the steps below. This somewhat tedious procedure is only necessary if you've confirmed that no administrator account exists on the system. Please take these steps to restore administrator privileges to your account.