The recent 19.10 version of Ubuntu MATE comes with its own variant of a notification daemon you could already install in older versions, with more features. One of these features is do not disturb.
Make sure if you are using 19.10 that you hadn't inadvertently kept this setting on! Else, you'll be tearing your hair out on why the pin for your Bluetooth keyboard isn't showing up.
If for some reason you prefer this mode to be on, then check your notifications for any Bluetooth pairing requests!