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Depending on what else may be wrong with your Mac mini computer, if the unit may have hardware failure or may
need to restart in "macOS Recovery" to try and reclaim a workable macOS (and any storage capacity lost due to
BootCamp issues, if a windows system may have corrupted the hard drive, etc.)
You may be advised to consider an authorized Apple service provider to perform diagnostic and repairs to the unit.
Sorry to to offer limited ideas right now, it's very early & I'm not at a Mac. And using an iPad without tech bookmarks.
You may find the reinstall macOS support article helpful, as it tells what keyboard shortcuts to use to restore or install
(and reformat) the Mac's hard drive. A TimeMachine backup and other backups or full clones may be helpful in this.
In any event...
Good luck & happy trails! 🙂
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