Try the following:
- Close Syntorial
- Disconnect any MIDI keyboards that are attached to your computer
- Restart your computer
- Without opening any other applications, open Syntorial
- 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 customer support via the Submit A Request link at the top of this page.