Happened to me once when I put shorty headers on my truck. In my case a portion of the bolt was still visible outside the header only the head of it was rung off. I soaked it with 2 cans of PB Blaster and let it sit for 24 hours. Next day I took a pair of vise grips another can of PB and it turned right on out.
2004 Mustang GT
Sonic Blue Auto w/ Full Bolt On's
Quarter Mile - ?? Never Run