It is an infuriating problem that your Mac won’t start up. As we know, a new MacBook has been released for some time, and maybe you already have it (More details about the new MacBook). So now comes the question: before using the new MacBook, you need to transfer your data from your old Mac to the new Mac. However, when you want to re-open the old Mac, you may meet a lot of questions that your mac won’t start up, then you feel at a complete loss.
So, what can we do to fix the problem? In this case, here are some good advice for you to do when you have a non-starting Mac.
1. Check your Mac turns ona. Check the connection to the power.
b. Try to use a different power cord or adaptor.
c. Unplug the useless cable or accessories (such as printers and USB hubs).
Note: If you recently installed new memory or a new hard drive, make sure they are correctly installed and compatible (if possible re-install the old memory or hard drive).
2. No video signal (or it's distorted)Try to reset the PRAM or starting up in Safe Mode.
a. Check the power supply to the Mac (and the power to the display if using a separate unit).
b. Confirm that all cables are connected securely.
c. Check that the monitor is compatible with your Mac.
d. Remove all display extenders, switches and any other devices between the Mac and monitor.
e. If using a separate monitor, unplug the video cable and plug it back in.
f. If using more than one monitor in a "daisy chain", unplug all monitors and test using just one.
g. If possible try to use a different display or a different adaptor (use DVI instead of VGA, for example).
3. Run Disk Utility in Recovery ModeYou can run Disk Utility by booting into OS X (macOS Sierra) Recovery Mode on a Mac running OS X 10.8 Mountain Lion or later.
a. Make sure the Mac is off.
b. Release the keys you were holding down when you see the screen with headlined OS X Utilities.
c. Click on your Mac's built-in hard drive in the left column of Disk Utility.
d. On the lower right of the Disk Utility window, click Verify Disk, and then wait while Disk Utility does its thing.
Besides, if you have an iPhone, and you need get your iPhone into recovery mode, here is another tip for you.
4. Safe Boot the MacSafe Boot limits what checks and functionality your Mac focuses on during startup, and performs certain diagnostics.
a. Shut the Mac down, and start it up while holding down Shift.
b. To get some feedback about what's happening, you might choose to start up while holding down Shift, Command and V.
Notes: If the Mac starts up normally, go on with your day. Otherwise, keep working through this list.
5. Consistency checks the File systema. Shut the Mac off, and start it up again while holding Command and S.
b. Wait until the command-line prompt appears, when all the text is done scrolling past. Then type fsck-fy and hit Return.
c. After five different checks that take varying amounts of time, you should get to one of two messages: "The volume appears to be OK" or "FILE SYSTEM WAS MODIFIED."
d. If you see the latter message, though, you'll want to run fsck-fy all over again. You can retype the command and hit Return, or press the Up arrow once and then press Return.
e. You'd eventually get to the "appears to be OK" message, then find that your Mac now starts up perfectly. However, if this doesn't work, and your Mac still doesn't start up, then move on to the next step.
6. Reset the NVRAMa. Hold down all of the keys: Command, Option, P and R, and turn on the Mac. Keeping holding the keys down until you hear the Mac restart again.
b. After performing this step, your Mac will restart normally. In other cases, you might instead see a progress bar on startup.
In another word, resetting that data isn't harmful, but quite frankly it's also rarely genuinely useful.
7. Reset the SMCa. Press Command-Option-Escape to force quit any application that is not responding.
Put your Mac to sleep by choosing Sleep from the Apple menu. Wake the computer after it has gone to sleep.
b. Restart your Mac by choosing Restart from the Apple menu.
c. Shut down your Mac by choosing Shut Down from the Apple menu.
d. If your Mac isn't responding, force the Mac to shut down by pressing and holding the power button for 10 seconds. You'll lose any unsaved work in any open applications.
See more: Resetting the System Management Controller
8. Target disk modea. Connect both Macs together using an Apple Thunderbolt cable (it also works with FireWire cables on older Macs).
b. Swift off your Mac (hold down the power button if necessary).
c. Start up your Mac while holding down the T button on the keyboard.
d. Keep holding the T button down as you hear the startup chime and keep it pressed until the Thunderbolt icon appears on your screen.
9. Reinstall Mac OS X or macOS SierraBoot into Recovery mode, and then click to install the latest OS and follow the on-screen prompts.
See more: About macOS Recovery
10. Make a Genius Bar appointmentTake your Mac to an Apple Genius Bar, or arrange for a repair under warranty. (If your Mac still doesn't work after doing the above steps). Besides, before you doing it, you must make sure you have got enough data from your Mac for backup, and if you have any other questions about how to recover apple data, read it and get help from it.
From above method, you can easily solve the problems when your Mac doesn’t work. Study it and fix it by yourself!