Chrysler PT Cruiser

I have a 2001 pt cruiser that overheated and blew a headgasket, warping the heads. I found a mechanic who swapped out the engine for a nice price.(Sarcasm). He has had the car for over 2 months. I had to make payment arrangements but he got 1/2 up front. I called him this week to see if the car was done, he claims I had the battery in backwards and my computer is fried. I can't check to see if the motor he put in is good because he says car won't run with bad computer. He told me after I finish paying for engine swap he will have the car towed to my house and I can take it in and get computer repaired elsewhere. I never had the battery out of the car. How much damage can be done to my computer if someone put jumper cables on backwards?
October 17, 2007.

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