You're on the right track but if disconnecting battery hasn't "rebooted" the player, you may need to take it to a car stereo shop or the dealer.
There's a "pull the hot fuse" trick but it sounds a bit risky. Details are in the How to Remove a Stuck CD guide if you get desperate and the other suggestions don't work.
Good luck, and please report back using the Answer this Question button here when you or a shop figures it out. Or post a comment at the guide. Thanks!