No, You're Not Alone (Avengers/Supernatural...
Tony Stark goes missing after Civil War, Rhodey is worried, but nobody seems to realize how serious he really is. Everyone just tells him that Tony is probably hiding, nobody listens, but Tony is still missing.
A while later, the Winchesters kill a witch that had a captive. A captive named Tony, whose puzzling memories are all violent and scary. He doesn't want to go back, so he decides to stay with them, and becomes a Winchester.
Time passes, and the Avengers realize their mistake. But will their search bear fruit, will they be able to find their lost comrade? Would he want to be found, when he already has a new family, one that never hurt him as bad as they did?