Q: I downloaded El Capitan and now I can't turn on my MacBook. When I switch it on it simply switches itself off

Hi preciousfromjohannesburg,
 

Thanks for participating in the Apple Support Communities. 
 

If you can't turn your MacBook on after installing El Capitan, here are a few steps to try: 
 

Try Safe Mode

If you're using Mac OS X 10.2 or later, you can start up your computer in Safe Mode, which includes an automatic disk check and repair. If you're using Mac OS X 10.1.5 or earlier, skip to the next section. A Safe Boot, which starts up your computer into Safe Mode, might allow you to start up your computer successfully. To do this, follow these steps:

1. Start up in Safe Mode.
2. After the system has fully started up, restart your computer again normally.

 

If your Mac successfully restarts, the issue should be resolved. If the issue continues, try using Disk Utility.

Try Disk Utility

Use these steps to use the Disk Utility app to verify or repair a disk.

1. If you're using OS X Lion or later, start the computer from Recovery System or Internet Recovery. If you're using an older version of OS X and your computer came with a Mac OS X Install disc, insert the installation disc and restart the computer while holding the C key instead.
2. When your computer finishes starting up, choose Disk Utility from the Utilities window, or from the Installer menu if you're started from an installation disc.
3. Click the First Aid tab.
4. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
5. Select your Startup Disk (usually named "Macintosh HD").
6. Click Repair. Disk Utility checks and repairs the disk.

You can find these steps and more information in the troubleshooting article Resolve startup issues and perform disk maintenance with Disk Utility and fsck - Apple Support. 
 

Best Regards.