Because you've said so little about the computer in question, such as build year &/or
version of macOS, suggestions to help are also vague; so inaccuracies may persist.
Sometimes a restart in Safe mode can help troubleshoot hardware or software issues.
•Use 'safe mode' to isolate issues with your Mac - Apple Support:
https://support.apple.com/en-us/HT201262
Also, displays require correct display adapters and cabling to perform as intended. And
some won't work if they're not the type intended to work in tandem other hardware.
What kinds of cable/adapters have you tried, and did they come from an Apple source?
If you were to re-set NVRAM on your Mac mini (any build model) certain results
should occur; if the process were enacted properly with follow-through.
• Reset NVRAM or PRAM on your Mac - Apple Support
To reset NVRAM, first turn off the firmware password. When your Mac finishes starting up, you
might want to open System Preferences and adjust any settings that were reset, such as sound
volume, display resolution, startup disk selection, or time zone.
https://support.apple.com/en-us/HT204063
Another reset that does different things, could (by some stretch) be helpful in troubleshooting;
however only if these are done to completion and correctly implemented. ~ The SMC reset has
certain attributes that can be useful to troubleshoot. This is akin to 'power management' reset.
How to reset the System Management Controller (SMC) on your Mac
Follow these steps for iMac, Mac mini, Mac Pro, and Xserve.
- Choose Apple menu > Shut Down.
- After your Mac shuts down, unplug the power cord.
- Wait 15 seconds.
- Plug the power cord back in.
- Wait 5 seconds, then press the power button again to turn on your Mac.
https://support.apple.com/en-us/HT201295
[A newer Mac computer with T-2 chip may require other steps to be certain this works.
Note the section in SMC reset article that applies, if your Mac Mini is latest 2018 model.]
Good luck & happy computing! 🙂