I don't think there's any chance for long-term success with a head bolt missing. IMO, you should pull the head back off, get a new set of bolts and reinstall. It sounds like maybe there was coolant in that block hole, which would leave no place for the bolt to go. The other possibility would be cross-threading, but that's not nearly as likely. If it's not cross-threaded, you can drill with left-hand drill bits, and use a large shank screw extractor to remove the broken bolt. DO NOT try to force the extractor if it's not turning easily. Have it towed to a pro who does this stuff every day. And who knows, maybe you will even get lucky on this one and have enough bolt sticking out to grab onto and just turn it out.