If you can reach it with some Vise-grips, you might be able to clamp down on it and twist it out. But it will also chew up the bolt head so watch out. Otherwise, it's time to drill and extract.

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'06 X5 4.8is, TiAg/black, cold weather pkg., comfort seats
'07 750Li, TiAg/black, sport pkg.
'05 Ford F350 silver met./slate gray, Lariat, 6.0 PSD, CC, DRW, FX4