Question: Q: MacBook Pro Won't Boot, Arghhh

Could be the ribbon cable connecting the drive to the motherboard needs replacing. Also, see My Mac only runs properly in Safe mode.

About Using Safe Mode

• Use safe mode to isolate issues with your Mac - Apple Support
• Playing Safe- what does Safe mode do?

That means you need to erase the drive and install macOS from scratch.

If you have not upgraded your firmware to implement Internet Recovery, then visit this link: MacBook Pro (15-inch, Early 2011). If you can do an Internet Recovery:

Internet/Network Recovery of El Capitan or Later on a Clean Disk

     If possible, back up your files before proceeding.

1. Restart the computer. Immediately, at or before the chime, hold down the (Command-Option-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the target drive (out-dented entry w/type and size) from the side list.
5. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs, only if installing Mojave or Catalina ) or Mac OS Extended, (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Install OS X and click on the Continue button.

If you cannot do Internet Recovery, then you need to be able to install the original Snow Leopard that was pre-installed when you purchased the computer. You should have two Software Restore DVDs with gray labels. Or if you have a bootable Lion DVD or USB flash drive, then either can be used.

Factory Reset Your Pre-Lion Mac Follow these instructions until you get to Step 6 of Factory reset of your Mac - Apple Support. At Step 5 you will need a Snow Leopard DVD or the installer disc that came with the computer.

1. Boot the computer using the Snow Leopard Installer Disk or the Disk 1 that came with your computer.  Insert the disk into the optical drive and restart the computer. After the chime press and hold down the  "C" key.  Release the key when you see a small spinning gear appear below the dark gray Apple logo.
2. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities' menu. After Disk Utility loads select the hard drive entry from the left side list (out-dented entry - mfgr.'s ID and drive size.)  Click on the Partition tab in the Disk Utility main window.  Set the number of partitions to one (1) from the Partitions drop down menu, click on Options button and select GUID, click on OK, then set the format type to MacOS Extended (Journaled), then click on the Apply button.
3. When the formatting has finished quit Disk Utility.  Proceed with the OS X installation and follow the directions included with the installer.
4. If you are planning to sell or give your computer away, then do the following: After you reformat your hard drive and reinstall OS X, the computer restarts to a Welcome screen and asks you to choose a country or region. If you want to leave the Mac in an out-of-box state, don't continue with the setup of the system. Instead, press Command-Q to shut down the Mac. When the new owner turns on the Mac, the Setup Assistant will guide them through the setup process.