Sometimes your Mac's parameter RAM (PRAM) or Non-Volatile RAM (NVRAM) gets screwed up and can cause start-up problems for your computer. Turning off your computer's power doesn't clear PRAM and NVRAM; a battery or capacitor on the motherboard keeps them active. If you think you’re having this problem, here’s what you need to do:
To reset — or “zap” — the PRAM/NVRAM, restart your Mac and press Command+Option+P+R until your Mac restarts itself. You’ll see the spinning-disc cursor for a few seconds, then the icon disappears, and your Mac chimes again and restarts.
After resetting the PRAM/NVRAM, you may need to reconfigure settings for speaker volume, screen resolution, startup disk selection, and time zone information. (By the way, there are technical differences between the PRAM and NVRAM and they depend on what model of Mac you have. But both can be zapped with the same procedure, and the results will be the same.)