mac mini a1347

Start up from macOS Recovery

To start up from macOS Recovery, turn on your Mac and immediately press and hold one of the following combinations on your keyboard. Command-R is generally recommended, especially if you never installed macOS Sierra 10.12.4 or later… How to reinstall macOS - Apple Support

Command (⌘)-R

Install the latest macOS that was installed on your Mac.

Option-⌘-R

Upgrade to the latest macOS compatible with your Mac.

Shift-Option-⌘-R

Install the macOS that came with your Mac, or the closest version still available.

But it sounds like you have FileVault, (Disk Encrypted), turned on & if so we'll get nowhere without the password.

Depends, is it a FileVault password? If so you can't without a password.

How are you trying now & where do you get stopped?

It would be CMD, but if a PC USB KB then...

Win key = Mac CMD key.

alt key = Mac Option key.

If FileVault was turned on, you simply won't get anywhere without the password, it was invented to protect your data from anyone without the password

Best of luck my friend.