Not good...sounds like the threads in the crank or bolt were bad somewhere and it got cross threaded. That would cause it to bind and then snap.
It may be possible to drill a small hole in center of bolt then try and use a bolt extractor. That may not work since the bolt has siezed in crank. I have broken extractors at work on siezed bolts so its not a fun job to do.
If you cant do it you may want to find a machine shop. They may be able to get bolt out and retap crank. This may your best bet and possibly less headaches.
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