He is lost forever!
Tossed there by a boss? We had that happen to elf after chaos spawn had eaten her.
Exactly the same thing happened to us.
We had our Dwarf get grabbed by the Chaos Spawn and ended up in the attic of the barn with no way out.
He got out when the Chaos Spawn came back, grabbed him again out of the attic and proceeded to gnaw on his brain.
You think THIS is tragic? Kruber managed to free himself from bondage and THEY BROKE HIS LEGS! HE WILL NEVER WALK AGAIN!
That place is more dangerous than whole chaos/skaven pact! I got stuck too yesterday.