2012 MacBook Pro will not start up. The chime works, the screen lights up, the apple logo appears, the bar appears and the black line appears and moves left to right to almost half way across and then everything quits and returns to a black screen.

Can you boot the Mac in Safe Mode? See Mac startup key combinations - Apple Support. Also, does your Mac have its original HDD or has it been replaced?

If the Mac does not start up properly in Safe Mode,

• does it boot in macOS Recovery - About macOS Recovery on Intel-based Mac computers - Apple Support, or,
• can you run hardware diagnostics - Use Apple Diagnostics to test your Mac - Apple Support?

Hi,

Resetting SMC could solve the issue.

How to reset the SMC of your Mac - Apple Support

If it doesn't work, try to make Genius Bar reservation if your local retail Apple Store is open or find local opened Apple Authorized Service Provider to be examined your MacBook Pro there.

Genius Bar Reservation and Apple Support Options - Apple

Apple - Find Locations - Location Selector