You say that the whole water pump broke off??
Wow! That may be the first time I've heard that one on here. While pictures will surely help, my first guess would be that your pump froze up and the squealing sound was it'* last death throes. When you revved it, you may have applied enough torque to a frozen water pump pulley to break a faulty casting.
Anyway, that'* just a guess based on your first description. Pictures would really help!
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Mike: 1997 SE w/ 183K, aka White Horse
, Gutted Airbox w/K&N, drilled 180* t-stat, tint all around, clear corners w/SilverStars, "PONTIAC" light up tail, PEMs, NGK TR55s