Q:

Whole Disk Encryption and power failures

I believe encryption softwares like TrueCrypt and VeraCrypt support whole disk encryption.
Now lets say I open my computer give my decryption password and successfully boot and am using my computer. Lets say the power suddenly goes off, and the computer shuts off. What happens then ? 
Correct me if I am wrong, the contents on my hard disk are still in the decrypted format right ?
Or lets say while encrypting the computer crashes due to a power failure or a hardware failure, what happens then ?

A:

No, the disk contents remain encrypted even while in use – the OS decrypts sectors in memory. (Just like in the 2nd question, you noted that encrypting the disk takes a long time. If the OS had to fully decrypt it on each boot, that would also take just as long – not to mention being useless.)
Most full-disk-encryption software keeps track of the current encryption progress (inside the encrypted volume header). After a power outage, there shouldn't be data loss since TrueCrypt can just resume from that point – or at least no more than a few sectors at the boundary.

