We need more details...
Did the bolt break while the engine was running? In that case you have valve and lifter damage at the very least...
Those bolts are known for working their way to becoming loose and the sprocket often damages the nub on the crankshaft... If this is the case... depending on the damage to the crank... you may wish to replace the engine if you can find a decent used one...
If the bolt simply sheared while you were changing it, it's not all that tragic... once there's no more clamping force on the sprocket, the rest of the bolt should turn fairly easily and as long as you don't get too cheap an easy-out, you should be able to turn it out...
Good luck,
Yuri