The update to macOS Monterey version 12.3 may have caused you problems if you connected a non-Apple monitor to a Mac mini, for example. Many use 4K monitors and it is possible that these (when booting) no longer display a picture or only one works even though you actually have a dual monitor setup.
The reason seems to be that Apple did not correctly implement the DisplayPort 1.4 standard in macOS 12.3. 4K monitors that rely on DisplayPort 1.4 are completely normal. The LG 49WL95C or the LG 27UL850-W could be mentioned.
LG 49WL95C, 27UL850-W and other 4K monitors under macOS 12.3
But there is a solution to the situation: using the DisplayPort standard 1.2. This is also fast enough for 4K monitors and continues to work under macOS. To use this trick you have to go through the menu in your monitor settings (in the monitor, not in macOS) where you can set the DisplayPort standard. Just set it to 1.2 here and everything should work fine like before updating to macOS 12.3