The most common reason to snap a cam is a weak cam, or the cam bearings were not in properly. This can be to a bad cam line bore, or improper installed bearings. But it could be just a bad cam casting.
The cam can also snap due to extream stress such as a stuck valve, but this usually bends the pushrod instead.
Once it snapped the engine might be okay, depending on the twist transferred to the block.

Good Luck