Q: Macbook Boots to black screen with stop sign !

Try Safe Boot:

- Shut down your Mac

- Wait until your computer turns off and after that press the Power button

- Right after you hear the startup tone, press and hold the Shift key

- Release the Shift key when you see a grey Apple sign and the progress bar below this sign

- After your Mac boots up, restart it as you usually do.

If this doesn't help, follow the instructions below:

- Shut down your Mac

- Wait until your computer turns off and after that press the Power button

- Right after you hear the startup tone, press and hold the Shift key

- Release the Shift key when you see a grey Apple sign and the progress bar below this sign

- Once you see Desktop, start a Disk Utility scan to detect and repair file system errors (don't forget to choose your main hard drive)

- Click on Verify Disk and then, if asked to fix problems, on Repair Disk

- After this, click on Verify Disk Permissions and then on Repair Disk Permissions

- After the process is finished, shut down your Mac and turn it back on after about 30 seconds

Apart from that, take a look at this Apple article and follow the instructions on Resetting NVRAM shown there: How to Reset NVRAM on your Mac - Apple Support

If these steps don't help, reinstall OS X. Make sure you are connected to the Internet. After that, follow these steps:

1. Restart your Mac. Once it restarts and the grey screen appears, press and hold the Command and R keys.
2. Select Disk Utility and click Continue.
3. Select your startup disk on the left and click the Erase tab.
4. Choose Mac OS Extended (Journaled) from the Format menu, enter a name, and then click Erase.
5. Select Reinstall OS X, click Continue, and then follow the instructions on your screen.

If these steps don't work, contact Apple Support:

https://www.apple.com/support/macbookpro/contact/

Look at the appropriate section in this support article:

https://support.apple.com/en-us/HT204156

Ciao.