Mac Mini won't wake up after 12.3 update

i would try going into System Preferences > Energy Saver and put a checkmark in "Prevent your Mac from automatically sleeping when the display is off" and remove the checkmark from "Put hard disks to sleep when possible" and see if that corrects the behaviour.

but even if that works, it may just be a "band-aid". i'm thinking you may want to download and run the free version of EtreCheck so we can see if you have some conflicting software installed that is causing your issue. make sure you give "full disk access" to etrecheck. read how to use it by reading Using EtreCheck. if you need help interpreting the report, you can see how to post the report here by reading How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report. and it automatically obscures sensitive things (like serial numbers) so you don't have to worry about sharing the report here.

I had the same problem and solved it by changing the monitor settings: To DisplayPortVersion = 1.2. See:

https://www.mactechnews.de/forum/discussion/Problem-mit-LG-49-Zoll-nach-Upgrade-auf-12-3-346283.html

The 12.3.1 update seems to have solved the issue!

I had the same issue with a Benq EX3501R connected via USB-C. Changing the Display Port setting on the display from 1.4 to 1.1 (my only alternative option) didn't solve the issue. I had to connect to the display via HDMI.

This issue is widespread and does not appear to be related to any of the suggestions offered in any of the replies above. The two main things that I have discovered and which may be of use to Apple's developers are:

1. When the Mac is first booted up, clicking on the Apple icon --> About --> Display shows the correct display information but, if you go to Settings --> Displays. you will get a Preferences Error (Could not load Displays preference pane.)
2. If the Mac then puts the display to sleep, then, when you wake it, the display receives no input because, if you go to the Apple icon --> About --> Display, it has lost the real information and replaced it with 'default internal display'! Perversely, Settings is now able to load the Display preference pane, which, at this point, is of no help.

I'm pretty sure that Apple with have a fix for this soon.

Similar issue here: 2018 Mac Mini with an HP U32 4K HDR display, which used to work fine over USB-C. Now with 12.3, the Mac won't even finish booting after entering password with the screen connected via USB-C. Same display using HDMI, and all works fine.

I hope so. My solution is to use a screensaver to quickly start. If the monitor and computer separate at any time, sleep/shutdown I can't re-boot without a start/fail to send info to monitor/re-boot to get going😕. I even must remember to turn on the monitor first, then the computer, or it won't boot.

I have also discovered that if I boot my Mac headless, and then connect the TB cable thereafter, the Mac does not recognize the monitor at all. The monitor receives no signal and the Mac does not have any knowledge of the connected monitor ( confirmed by logging in to the Mac in ARD). The Mac therefore only sees the connected monitor if it is present at boot, and will not reconnect on waking up from sleep nor if you plug it in post-boot. This would appear to be related to some driver issues introduced with the latest update. Can anyone else confirm this behaviour?

I encounter the same issue since updating to 12.3.

This problem occured some time ago and was fixed in a previous system update.

A shame it is coming back ...

macminiperson wrote:

No hard disks, just an SSD so that second line does not appear on my machine.

that is another indication that your issue is software related. my iMac has 100% flash storage, and that same option is present on my mac. and i keep it de-selected even though it does nothing for an SSD. as far as the validity of etrecheck, just look through these pages. it gets recommended and used here countless times every day. it greatly helps us to help others. do you really think the moderators would let us continue recommending it if it was nefarious? :)

and you are right to be cautious about which software you put on your mac! that is the BEST preventative measure you can use. and i may kind of sound like i'm pushing it. but it is, IMHO, the best diagnostic tool we have at our disposal. it can point out issues without having to play "20 questions" with our fellow users. it was written by a very respected contributor to these forums. all you need is the free version. and i need to add that i get absolutely ZERO compensation for recommending it. i just value it enough to recommend it. and i also keep it on my mac. i run it whenever i add new software to my mac to ensure i'm not getting something i don't want bundled in with the new software. wether or not you choose to get it is, of course your decision. it is your machine after all. but it kind of limits our ability to help you.

and, not to preach, but, are you powering down the mac and the display when removing and plugging back in those cables? if not, that's a really bad approach for A/V equipment. most times it does not cause an issue, but occasionally that method can and does damage electronics.