why do I get a black box flickering on the left of the screen when I start my macbook pro

If you can't boot in the usual way, try a safe boot.

During startup, you’ll see a progress bar, and then the login screen, which appears even if you normally log in automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin. If you have more than one user account, you must be logged in as an administrator to carry out these instructions.

Safe mode is slower than normal, and some things won’t work at all.

Note: If FileVault is enabled on some models, or if a firmware password is set, or if the startup volume is on a software RAID, you can’t boot in safe mode.

If you're able to boot, launch the Console application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad. Click Utilities, then Console in the icon grid.

Select the most recent panic log under the heading System Diagnostic Reports on the left. If you don't see that heading, select

View ▹ Show Log List

from the menu bar. Post the entire contents of the panic report — the text, please, not a screenshot. In the interest of privacy, I suggest you edit out the “Anonymous UUID,” a long string of letters, numbers, and dashes in the header and body of the report, if it’s present (it may not be.) Please don't post shutdownStall, spin, or hang reports.

how do i know which to post and post the panic report