Stereo doesn’t play after a battery reset?

Many manufacturers have implemented an anti-theft mechanism in factory radios where you have to enter a code into the radio after power has been removed from it to discourage thieves; if you can't use the radio you've stolen then there's no point in stealing it.

Generally the radio code will be included with any documentation received when the vehicle was purchased; I'd look in the glove box for an owners manual and any papers that might be with it. My Volvo and my Volkswagen both list four digit codes that have to be entered whenever I disconnect the battery. Your owners manual may also tell you how to enter the code to reenable the radio.

Sounds like a possibility of what's happening with your radio; as a last resort your local dealer should be able to provide the radio code if you don't have it as long as you can provide proof of ownership.