If you use the past participle of the word as an adjective it is a reduction of passive usage.
Normally the bed which you did not make or the bed which wasn't made is the unmade bed. According to the situation you can use it before or after a noun:
I found all letters unwritten
I found all unwritten letters
For the adjective nothing changes, but there are exceptions here. Most of them can be used with or without a noun, but for some you should change it:
The man was drunk.
The drunken man fell down.
The ship was sunk.
The sunken ship was very big
You can find further explanations on the Internet...