A computer consists of many components that all work together, sometimes they do not communicate well with each other.
A simple reset often solves many problems. Rest assured, you will not delete files with this, so you won’t lose anything with this reset.
Follow these steps:
Turn off the computer completely.
(While the computer is off, press the OPTION + CMD + P + R keys simultaneously.
Now turn on the computer and still hold these keys all down!
The computer restarts, you still keep pressing everything, it will now turn itself off and on again.
Do not release the key combination until you hear the startup sound for the second time.
You are done!