English word for something left and forgotten?

Solution 1:

The word that seemed most appropriate to me when I read this was languish:

1. to be or become weak or feeble; droop; fade.
2. to lose vigor and vitality.
3. to undergo neglect or experience prolonged inactivity; suffer hardship and distress: "to languish in prison for ten years."
4. to be subjected to delay or disregard; be ignored: "a petition that languished on the warden's desk for a year."

Solution 2:

Perhaps, it has been left to molder?

For me, it carries connotations of something that has been intentionally abandoned to progressive decay.

Here's a definition taken from Google:

slowly decay or disintegrate, especially because of neglect