Hello bbgreene, if the keyboard has a lightning port, hard wire it and make sure it is turned on (slider switch). If the other type of keyboard that takes 2x AA batteries, you will need to hold the power button on the side until it turns on.
If all else fails, try performing the following actions.
choose Apple menu > System Preferences, then click Bluetooth . Select the keyboard, mouse, or trackpad in the Devices list, then click Connect