FileVault takes quite a while to encrypt and decrypt your Home folder.
It's really a bad idea to stop it once you have it going in either direction.
This can take half a day or longer depending on how much data you have stored.
Before you end up with a serious data loss issue, I would burn everything important to CD or a back-up drive.
Don't forget your Address Book, Bookmarks, and all your Home folder items.
If FileVault fails to decrypt your system after you've saved everything, you may end up needing to run a clean install so make sure your data is
backed up off your computer.