In the PowerPC days, we talked about resetting the PRAM. On modern Macs, the real term is resetting the NVRAM. The name refers to special memory sections on your Mac that store data that persists even when the Mac is shut off, such as volume settings and screen resolution.
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Resetting that data isn't harmful, but quite frankly it's also rarely genuinely useful. But it can't hurt.
You might need to grow an extra finger or two for this one, or have a friend help you out. Hold down all of these keys: Command, Option (Alt), P and R, and turn on the Mac (it's the same keys to reset the PRAM). Keep holding the keys down until you hear the Mac restart again. Apple says to let it restart just the one time; I usually listen for a second reboot, and then release the keys.
In some cases, after performing this step, your Mac will restart normally. In other cases, you might instead see a progress bar on startup. If the progress bar fills up and then the Mac starts up, you're probably good to go. In some cases we've seen, however, the Mac shuts down at around the halfway point in the progress bar.