Sometimes a simple reset of the System Management Controller (SMC) and NVRAM (a small amount of memory that your Mac uses to store certain settings and access them quickly) can easily resolve many unexplained issues you may be experiencing with your Mac.
Read all steps start to finish first so that you have an idea of the various keys you'll be pressing down and the timing of each step -- and then follow each step in order:
1. Shut down the computer. You may have to do perform a hard power-off by holding the power button down until the MacBook shuts off.
2. Unplug the power adaptor from the side of the MacBook and unplug any other peripheral devices plugged into the MacBook (headphones, USB devices, external display)
3. Using the built-in keyboard, press and hold the Shift, Control, Option and Power buttons for 10 seconds, then release all keys.
4. Reconnect the power adapter
5. Press the power button and then immediately hold down these four keys together: Option, Command, P, and R. Keep holding the keys for about 20 seconds, during which the MacBook might appear to restart several times. Release the keys after 20 seconds (on older Macs, you will hear multiple chimes during this process. Newer Macs will not chime at all).
6. Verify successful boot to the MacOS login screen and ensure you can log into your account.
7. Reconnect any remaining peripheral devices.