Mid 2012 MBP thunderbolt not working after updates

Hey there Rupert,

We'd like to help get your external monitor working as before, so let's start with ruling out some possible causes:

1. You mentioned updating the memory, along with the OS version. Please confirm which OS version number shows after clicking on the Apple logo at the top left corner of the screen, then on 'About This Mac'.
2. Let us know the results from testing another monitor with your Mac, or even testing the same affected monitor with another video source.
3. Keep us posted on the results from the steps here: Get help with video issues on external displays connected to your Mac

We eagerly await your response about these three details, and take care until then!

Hello Rupert,

While the latest version is macOS Big Sur 11.0.1 as it was recently released, thanks for confirming you have the latest version of macOS Catalina. That's also good to hear the same monitor and connection have no issues with that other Mac you tested. While you may need to get in touch with Apple Support, let's continue ruling out what we can:

1. Are we correct saying that you've completed the other steps in the link that was sent? Here it is again just in case: Get help with video issues on external displays connected to your Mac
2. While you have the latest macOS Catalina version 10.15.7, please check for any new updates. Sometimes, there are security updates that don't necessarily increase the version number.
3. If you've already completed the steps in the link, along with making sure there are no other system updates, the next step would be to get in touch with Apple Support here: Get Support

Have a good one!