12 disc cd changer not working after car sat for a while on "96 TransAm

It is not the theftloc I think. It worked when I parked it.I hit play and it only goes to track 12.Sometimes I can get it to go to track 11. It won't read any discs, even a laser cleaner.Error code E31
displays.Alldata code description is cannot read disc.Dealer no help,couldn't find any TSB's or other info. A friend of a friend said he had similar problem and is common on these cars after battery disconnected or dead. Unfortunately he can't remember how he fixed it. Any info much appreciated.
Thanks guys

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