Radio won't work after battery change

I just changed the battery on my 2000 Grand Prix. The radio has "theftlock" written on the front, but I never activated it (although it may have been activated prior to me buying it used in 2002). After changing the battery, the power button for the radio will not work, thus the radio will not turn on; however, I am able to change the time. The manual says that if is it locked, then "loc" will appear on the radio display. The only thing that appears is the time. Any suggestions?