Charging restart
Connect the stereo system to the power supply and charge the battery. If the battery is overdischarged, it may take several hours to be fully charged.
Wait for flowers
If the power shortage is not serious, give the battery some time and it may recover itself. The waiting process may be a bit long, so be patient.
Warm embrace
Try to put the battery in a warm environment, such as near heating or sunshine, to make it feel warm and fully charged. But be careful not to overheat the battery.
Replacement plan
If all the above methods fail, a new battery may be needed. Some audio batteries can be replaced by themselves, or you can find a professional maintenance master.