I broke one of the bolts that holds down my campshaft journals. It broke right at the top of the threads. this really sucked because that bolt is very critical and it is also a grade 12 bolt, which means it is about as hard as any drill bit. if look at the cap of that bolt if it is a grade 8 bolt you can probably use some sort of "easy out" set to get it out. what worked for me was that since the bolt was so hard it broke with a very jagged end so i was able to use some PB blaster and the other half of the bolt to catch it and work it out. and once it cleared the threads i was able to get it out w/ a magnet.
but yeah they make sets of bits for getting out broken bolts and such. (here's one in MSC) just check at sears or any hardware store. how hard it is going to be to get it out depends greatly on how easy it went in. If you could turn with your fingers all the way in, it shouldn't be to bad to get out, cause as long as it didn't bottom out, there is really nothing holding it in.
95% of the time if you over-tourque a bolt, you are just going to stretch and break the bolt.