If you're referring to a bolt that holds the oil pan to the block, you need to do this:
1.) Remove the oil pan
2.) Remove the broken bolt, there's tools that you can drill into them to give you something to hold on to and twist out. Worst case scenario you have to drill out the hole and re-tap some threads.
3.) Clean up all FIPG (gray stuff) on the oil pan and block
4.) Use new FIPG (gray permatrex, or stuff from Toyota) in a small constant beaded line around the oil pan mating surface and around all the bolt holes
5.) Bolt the new oil pan on using proper TORQUE SPECS, I think it's 15 ft. lbs or something, not sure
It's annoying, but something you'll have to learn when working on cars. Taking the extra time to use a torque wrench beats the extra couple hours +/- of removing a bolt with a broken head.
It may not leak right away, maybe never since the FIPG probably has it glued on and it's only 1 bolt. Over time though, the seal may break free since the bolt isn't holding it or crack and you may experience leakage.