* Sunk is the past participle of the verb "sink". It's used to describe a state of being submerged or having gone down.
Example:
The ship sunk to the bottom of the ocean.
* Sunken is also a past participle of "sink", but it's more often used as an adjective to describe something that has been submerged.
Example:
The sunken treasure was finally recovered.
So, while both "sunk" and "sunken" are grammatically correct, "sunk" is more common in most cases.