Black screen. Cannot restart

Please restart your computer in Safe mode by taking these steps:

- Click on the Apple sign() at the upper left corner of the Desktop, select 'Shut down' from a drop-down menu;

- Wait until your computer turns off, then press the Power button(⌽);

- Immediately after you hear the startup tone, press and hold the Shift(⇧) key. It should be held as soon as possible after the startup tone, but not before the tone;

- Release the Shift(⇧) key when you see a grey Apple sign() and the progress bar below;

This will force the system to perform several maintenance routines and load essential system software.

Once you've seen the Desktop, start a Disk Utility scan to detect and repair file system errors:

- Open Disk Utility application:

1. Click on Finder at the bottom of your Desktop;

2. Select Applications folder from Finder's left sidebar and then open Utilities folder (or press Command(⌘)-Shift(⇧)-U), double-click on Disk Utility icon;

- Choose your main hard drive from the left sidebar ('Macintosh HD', usually);

- Click on 'Verify Disc' at the bottom right corner, your Mac will perform a diagnostic test. Click on 'Repair Disk' if prompted to fix problems, or follow instructions given on the Disk Utility window;

- Once the routine finished, click on ""Verify Disc Permissions"", after it's done, click on 'Repair Disc Permissions';

- Wait until the process's finished and shut down your Mac, turn it back on in 30 seconds;

If the weather was hot and humid then probably may be because of humidity, condensation might have occurred inside the macbook. In this case you should carry your macbook in sealed tight plastic bag. Take it to the Apple store for diagnostics.

Hope this helps......