I reset the screen resolution on my Mac mini. Now the display is black and says resolution not supported. How do i reset my mini without a display?

First try safe mode to see if you can get to your System Preferences menu:

How to use safe mode

Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:

Apple silicon

1. Shut down your Mac.
2. Turn on your Mac and continue to press and hold the power button until you see the startup options window.
3. Select your startup disk, then press and hold the Shift key while clicking “Continue in Safe Mode.”
4. Log in to your Mac. You might be asked to log in again.

Intel processor

1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
2. Release the key when you see the login window, then log in to your Mac. 
3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window.

If that fails you may need to go into Recovery and reinstall your MacOS:

How to get into Recovery. Click on link below:

Apple - Support - Search