1997 Pontiac Bonneville

I have a 1997 Pontiac Bonneville 3.8L - 167,000 miles on it. Replaced engine in March 2005. Took car to have oil changed and 3 days later, started hearing knocking noise when driving. Couldn't place exact location of noise, so while idling in driveway - started looking around. Engine stopped suddenly. Now will not turn over. Mechanic is really no help - wants to replace the engine again. A friend who is a car repair buff loaned us his hand computer to see what codes would pull up. Nothing. The friend thinks that this could be a very simple fix, but is not health-wise able to help. Any ideas on what it could be?
June 7, 2006.

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