Make sure to get back on ASIO or WASAPI drivers in the "Audio/MIDI" tab. You probably switched to Wave or DirectSound as driver type. In more recent Music Maker versions, which are now using the Samplitude audio engine, these driver types can no longer produce any MIDI output when in pause/stop mode. This would apply to the on-screen MIDI keyboard, the piano roll of the MIDI Editor, and even external MIDI input. (You will still get sound when playing back MIDI sequences in the arranger.) With any kind of audio or MIDI recording workflow you'd want to be using ASIO or WASAPI anyway, since you'll need to ensure the latency times are low enough.

Hope this helps!