The very first thing which comes to our mind is to find the actual source of problem. There can be many things at fault such as cables, audio amplifier, or speakers.
Make sure that these are properly connected between speakers, stereo, and amplifier. The wires between the stereo and amplifier need to be disconnected, but there should not be any unplugging done between the cable which connects the amplifier and speakers. Check if the problem still exists then your amplifier must be at fault.
The stereo system may not work, if there is a loose or missing connection. All the cables of car stereo, car amplifiers, and car speakers need to be checked separately, to ensure they are properly connected. Sometimes the cable is torn, or exposed which can also be the reason for system to be defunct.
Sometimes if power is not supplied to the speakers, they will not produce sound.
These speakers which have been installed by the car company always tend to work fine with the store-fitted audio system. However if different speakers are installed, they might not work due to compatibility issues, or may need more power to operate.
Also a blown fuse can also be one of the reasons for stereo to be nonfunctional. One should refer to the car owner's manual, to ensure all fuses in the box are intact. If they are fine, one needs to check if the car stereo systems have its own fuses. If found faulty replace them with new ones.
There is lot much probability that the speaker becomes faulty. The blown speaker can be checked by an electric meter.
To know whether your speaker is blown or not just place your hand over it and feel the vibration and thump which comes out at high volume. If in any case thump is not there then one should assume that the speakers are blown.
At an early stage if problems related to car stereo are diagnosed, it can then save a lot of time and money.