I have a new Mac mini

Turn the device off and on:

The Magic Keyboard has a switch on the back edge of the device. Slide the switch to turn off the device (no green is visible), then slide the switch back to turn on the device (green becomes visible).

Earlier Apple Wireless Keyboard models have a power button on the right side of the device. Hold the button until the green LED on the top of the device turns off, then press the button again until you see the green LED to turn the device on.

Make sure Bluetooth is turned on:

Using a mouse, click the Apple menu  > System Preferences, then click Bluetooth. Make sure that Bluetooth is turned on.

If the Bluetooth icon doesn't appear or if the menu bar status continues to indicate that Bluetooth is off, restart your computer and then try to turn Bluetooth on again.

Connect the Lightning cable:

Connect your wireless accessory to your Mac with the Lightning to USB Cable or USB-C to Lightning Cable, wait 1 minute, then disconnect the cable. This pairs the accessory with your Mac and charges the built-in battery. To check your accessory's charge level, click the Control Center icon in the menu bar, then click Bluetooth.

Earlier models don’t use a Lightning cable for charging, but have batteries that you replace. Learn how to pair those devices with your Mac.

Check for wireless interference:

If your accessory won't stay connected wirelessly, there might be interference with other wireless devices. Learn how to resolve Wi-Fi and Bluetooth issues caused by wireless interference.

See If you can't connect your Magic Keyboard, Magic Mouse, or Magic Trackpad to your Mac - Apple Support

What manufacturer’s keyboard are you using? Is it designed for Mac or Windows? When you say it is wireless, do you mean Bluetooth wireless or is it wireless via a USB dongle?