MacBooks come with a special type of memory that stores many low-level settings for your MacBook. Sometimes you need to restore the NVRAM to get your computer functioning again. NVRAM controls many functions, including the speaker volume, screen resolution and the startup disk selection.
After resetting the NVRAM, you may need to select the startup disk. This issue usually affects computers with more than one hard drive. Start the computer as normal and hold down the "Option" key as the computer starts. When the Startup Manager displays, use the left or right arrow keys to cycle between hard drive options.