Did the bolt break when you were trying to remove it? This is a common problem when mild steel fasteners are threaded into aluminum components. The two different metals in contact with each other tend to oxidize more quickly - the steel rusts and the aluminum corrodes. In many cases, coating the bolt threads with anti-seize when the bolt is installed can prevent problems when the fastener needs to be undone.
A good penetrating oil like PB (yellow can at Wal-Mart -$4) can go a long way toward dissolving the corrosion that can lead to broken bolts. If a bolt is frozen in place, and you haven't yet broken it, clean the joint so the penetrant has a chance to get down between the bolt and the hole. Then, tap the top of the bolt sharply with a small hammer, maybe a 6-oz. Spray again and wait for 30 minutes. The longer you wait, the better chance that the penetrant will be able to loosen the corrosion.
When you attempt to loosen the bolt, use a tightly-fitting socket or box wrench, preferably a six-point. Sometimes, working the bolt back and forth a bit, trying to tighten a little, then loosen a little will allow you to break the corrosion and get the bolt moving. This is a good time to spray some more penetrating oil.
As a last resort, heat can be used in situations where the components will not be damaged or fire is not a hazard. Heat the frozen fastener, then let it cool completely before spraying again with penetrating oil and attempting to remove the bolt. The expansion and contraction sometimes will break the bond of corrosion between the frozen parts.
Did the bolt break when you were installing it? If so, it will be much easier to remove than a bolt that is rusted or corroded in place. If enough of the bolt remains to get a good grip with a pair of vice grips, spray with PB, tap and try to undo it.
If the bolt is broken off flush and you cannot get a grip, then grind the top of the bolt flat and center punch it. Buy a few left hand drill bits. Start with 1/8" and drill as straight as you can through the center of the bolt. Increase the size of the bit until you get close to the size that is recommended for tapping. With any luck, the bolt will come out as you drill.
If not, you can try a screw extractor (EZ-out), but be careful! Don't try screw extractors on bolts that are corroded in place. If an extractor breaks off in the hole, the hardened steel is difficult to remove. If the extractor doesn't pull the bolt, without seeming like it might break, then drill the bolt out oversize and install a threaded insert (Heli-coil).
If the extractor breaks, you are reduced to grinding out the broken extractor with a tungsten carbide tool, drilling the hole oversize and installing a thread insert repair. Carbide bits are sold in many different shapes, so it may be possible to loosen the broken extractor enough to get it out of the hole. The hardened extractor must come out before you can drill the hole.
If all this seems daunting, a good repair shop or sometimes even a machine shop may be able to do the job for you. Broken bolts can be exasperating, and very expensive. The time and cost involved are a good incentive to be careful not to over torque when installing, and to be patient and careful when trying to remove a frozen bolt.