When I turn on my MacBook Pro, I see the Apple logo and the progress bar but then it turns off When I switch on my MacBook Pro 2011, I see the Apple logo with progress bar but before it enters the next screen it turns off. Why?
Troubleshooting is a process of elimination and can take quite some time. This means trying various things and reporting back with the results in order to try and get to the bottom of your problem.

Apple Hardware Test / Apple Diagnostics

The first thing to try is running Apple's built-in hardware diagnostics. To do this:


Ensure your MacBook Pro is connected to AC power
Start your computer
Immediately press and hold down the D key before the grey screen appear
When the icon appears on the screen let go of the D key
Select your language and click the right arrow
Select the "Perform extended testing" checkbox
Click the Test button
Take a note of the results that appear in the bottom-right window


Report back with your results.

NOTE: If holding down the D key doesn't work, you can also try holding down the OptionD keys during startup to try and run the diagnostics test via the internet.