How to stop file vault decrypting and re enable encryption? SHORT QUESTION : 

Ive got a macbook pro, late 2012, osx 10.10.5, with file vault enabled. 

Now when i go to System Preferences > File Vault i get the following decryption screen (see below screenshot). How can i stop it decrypting and re-enable deception ? 

LONG QUESTION (with background) : 

Ive got a macbook pro, late 2012, osx 10.10.5, with file vault enabled. 

When i tried to power it on earlier today it wouldn't start properly the screen was on, but very dull and it couldn't be made brighter using the brighter key. I waited a while but had to press the power button again to shut the machine down, when i powered it back up it took me through the forgot your password wizard. 

I hadn't actually forgotten my password, but it wouldn't let me do anything apart from go through the wizard. The forgot password wizard gave me 3 options : 1 + 2 where related to resetting the password and 3 was to do with disabling file vault, so i went with option 3. The computer then booted correctly. 

Now when i go to System Preferences > File Vault i get the following decryption screen (see below screenshot). How can i stop it decrypting and re-enable deception ?
Just wait for it to decrypt, then re-enable FileVault.