Volume keys are stuck
Your can't press your device's volume keys
Keys don't have any assignment
Go to your phone's settings and make sure that the phone's volume keys have been assigned with the desired function.
With time, every device's inside parts can get dusty if they don't receive the necessary cleaning. Improper cleaning is one of the major reasons why pushable keys get stuck, to fix this you will have to turn off your device and open the phone. Once inside you can clean the dust or remove the small obstructions by pushing the keys from inside.
Key's gel lost shape
If you take your phone with you to very hot places, it is possible that with time the gel that makes the volume keys go back to position looses its form. In this case you will need to buy new volume keys, open the phone and replace the keys using tweezers to remove the gel.
Instructions to replace the keys will be found here: Volume Button Replacement Guide
Device does not read SIM card
Your device is not detecting the SIM card
SIM card has been disconnected
It is probable that your SIM card has been disconnected for some reason. Try contacting your provider and using your SIM card in another phone to make sure the card is working properly.
SIM Card reader is not functioning
Before assuming any problems, try restarting the device and make sure you are in a place that receives good reception. If this did not fix the problem, then you should remove the SIM card and make sure that you put it back in place correctly.
If after trying all this the phone still does not read the card, then the phone's SIM card reader is not working properly. To fix this, you need to turn off your phone, remove the battery and open your device to replace the card reader.
Home/Return/Menu button stopped working
Your Home/Return/Menu button is unresponsive
Device is wet
These keys are prone to not work properly when they get wet or they are touched by wet fingers. Make sure that you dry your fingers and that your phone did was not in direct contact with water recently. In case your device was in contact with water you should turn off your device and wait for it to dry as using them when they are humid can produce internal damage tot he keyboard module.
Processor is being heavily used
When your device is put under heavy or numerous tasks at the time the processor can get overloaded. If the processor gets overloaded it will take longer for new requests to be processed, including you keyboard module keys. If this is the case you will notice a delay in the phone's response when you press the keys or even occassionate the software to crash. To fix this problem you will need to either restart your phone to give your processor a fresh start, or give your device some time to terminate the current processes.
Keyboard module is bad
If you've tried all of the above and the home buttons are still unresponsive then odds are the keyboard module went bad. If this is the case your Home/Return/Menu buttons will not respond when touched, you won't see anything on the screen nor feel the phone vibrate when the keys are pressed. You will need to buy a new keyboard module and open the phone to replace the old keyboard module with the new one.
Your speaker is not making sounds, or is making corrupted noise
Interference is being caused by another device
Every once in a while a phone can emit corrupted noise because of interference with another device. If you notice your phone emitting weird noises when you are in a specific location, or under specific conditions, you will need to either turn off the device that is creating interference with your phone or you will need to move your phone away.
Speaker is broken
Try restarting your phone before assuming physical damage. Sometimes when the processor is being heavily used, it receives more requests than it can handle, if this is the case your phone will emit distorted noises and to fix this you will need to clear your processors' tasks by restarting the phone.
If an overloaded processor was not the cause of the weird noises and your phone is not making any sound at all then we can assume physical damage.
The first step to take is to make sure that the device's speaker is placed correctly. For this you will need to open the phone and check if the speaker is moving or lingering. If this is the case you will need to press the speaker gently until you hear a click. If this is not the case then you have a broken speaker, you will need to replace it with a new speaker if this is the case.
Camera does not take pictures
Your phone's camera broke and can't take pictures.
It is very common for the a phone's camera to stop working when the software crashes, if this is the case the best course of action is to restart your phone.
Keep in mind that there are programs or events that cannot run at the same time as the phone's camera, make sure that this is not the case by closing every program that you have opened in your phone.
Lens is broken
Camera's lens can break after falling or receiving a hit. To resolve this issue you will need to replace the camera, first you will need to turn off your phone and remove the battery.