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BitDefender at best is worthless and at worst damaging to Mac OS. Please uninstall per the developers instructions. Mac OS is extremely secure and if left alone and simply kept up-to-date it will remain so and should be stable for many years. Mac OS does NOT require nor does it benefit from any third party antivirus, cleaning or any other "maintenance" apps.

First attempt to restart the computer by doing a NVRAM reset, instructions are in How to reset NVRAM on your Mac. If the computer restarts then please follow these instructions:

1. Click on Go from the menu bar and select Applications.
2. In the Applications window, you will see a Bitdefender folder (also indicated in the picture below). Open that folder.
3. In the Bitdefender folder, double-click on BitdefenderUninstaller.
4. A new window will pop-up. ... 
5. When the process is complete click on Close

If the computer will not restart after resetting the NVRAM, then please attempt a Safe Mode restart, instructions are in Use safe mode to isolate issues with your Mac - Apple Support and then do the steps noted above. After you have uninstalled BitDefender please empty the Trash and then restart normally and test.

I get the same message when I power on in safe mode. Is there another way to start up

I have encountered the same problem. I am a senior lady who thought I was doing the right thing by installing Bitdefender for Mac as additional protection. Obviously I was wrong. You’re above response just makes me feel even more stupid! And I would uninstall it....if I could get past the black screen with the error window!! For those of us who are not as tech savvy as you - a step by step bit of help would be much appreciated! I am unable to get past this black screen!