If you have enabled FileVault and the process is stuck, it's not trivial to revert the encryption. Under Settings / Safety the process can only be reverted, when the encryption has successfully finished. If you are using 10.9+ it's possible to revert the process via terminal. "diskutil cs revert /Volumes/[VolumeName] -passphrase" whereas [VolumeName] must be replaced by your hard disks name. If it is the standard "Macintosh HD", just type in "diskutil cs revert /Volumes/Macintosh\ HD -passphrase". The process will be reverted and your system should be responsive again.