Resetting PRAM or NVRAM is a useful first step to resolving login problems, fans running loudly, a computer that doesn't turn on correctly, volume issues, screen issues, unexpected shutdowns, and many more problems that may arise.
Shut your computer all the way down by clicking the apple icon in the top left corner of your screen and choosing "Shut Down..."
After 10 seconds with the computer completely powered off (no lights, no sound, no spinning wheel on the screen), press the power button again to turn the computer on.
IMMEDIATELY after hitting the power button, press and hold the following four keys at the same time
Continue to hold the above keys for at least 20 seconds or until you have heard a startup noise at least twice
While holding these keys, your computer should make a startup sound and flicker back off and on again. Continue to hold these keys the entire time. If your computer goes straight to the login screen without restarting first, shut it down and try again.