Try the following:

1. Close Syntorial
2. Disconnect any MIDI keyboards that are attached to your computer
3. Restart your computer
4. Without opening any other applications, open Syntorial
5. If you're using a MIDI keyboard, reconnect it to your computer. Then go into the Options menu -> MIDI and Audio Settings, and re-enable the MIDI keyboard.

If the issue persists, reach out to us via the Contact Us button at the top of this page.