This is the first step in resolving problems.
Reset the PRAM (Parameter Random Access Memory), this reset all connection Buses to default values.
This will fix some problems quickly.
Do this by restarting the machine (this applies to all makes and models).
As soon as the screen turns black press and hold the 'Option', 'Command', 'R' and 'P', they are marked in red in the picture.
Keep the keys down until you hear 4 startup chimes, this can be quick or it can be 10 - 15 seconds between chimes, wait for them.
After the 4 chimes then let go the keys and let it finish the start up sequence.