There are a number of reasons why a car stereo may not work. If a car stereo will not turn on at all, there is likely an issue with the wiring or fuses.
Many car stereos contain an in-line fuse behind the unit that helps regulate voltage to the stereo. In the event of a voltage spike, the fuse can burn out, preventing power to the stereo unit. Likewise, the wiring itself can overheat and burn. Other possible causes for car stereo failure are low wattage or a faulty antenna. There are many qualified stereo shops who can help diagnose and repair care audio systems.