Call asap and just be honest (do not email). I've done it once, I felt terrible, but I just held firm and said essentially "I'm sorry but this was unforeseen/unexpected, I'll try to send candidates your way, but best of luck with your search." Or something like that.
Hiring mgr let out a sigh and said something like back to the drawing board.
A year later a mutual friend who worked there asked if I was interested in per-diem work. So there's that.
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