I was working along in Dorico 3 just fine, then I unplugged my USB keyboard. Once I came back to work later and switched the USB keyboard back on, it didn’t work anymore in Dorico. So I looked in the preferences, and found the device wasn’t listed in the MIDI input preferences. And there doesn’t appear to be any “refresh” button.
I had to quit and relaunch Dorico to get it back.
Please consider this an improvement request for a future version of Dorico that, like other programs, it’ll auto-sense when a device is available or not, and not require a manual restart.