There are multiple reasons that can lead to a synth not making a sound.
Here are some cases and how to get yourself out of said pickle 
Check the Mixer
- Level for that Synth is up
- Check that it is not panned to a side that is currently not audible because of you console settings/cables/whatever
Check the Synth Menu:
- Local is ON for all Synths (when playing on the Grid)
- MIDI Channel is correct (when playing via an external source)
- Number of Voices is not set to zero
- MIDI clock is set to internal
Check the Sequencer Page:
- if it is Set to Seq it requires a sequence to play
- if it is set to Arp or Seq, it requires a clock, so try setting the clock to internal in the MIDI menu
Save the Scene to a new location so that you can go back, then init the Synth slot (SHIFT+PRESET → Init). If it works now, it must be one of the values stored in the sound. i.e. any parameter on the Engine, Filter, Amp, LFO or MOD pages, or a macro.
Reload the saved scene and check Synth Parameters, the most likely ones are
- Amp Envelope / Filter Envelope
- Filter Type, Cutoff, Env Amount
Also check if Mods and Macros affect these parameters.
Possible next steps:
- Init the scene
- Reboot the machine
- delete the workspace and/or global settings from the SD-Card
/workspaceis the last known state of the current scene (saved or unsaved). So be careful when deleting this as any unsaved changes will be gone afterwardsglobalSettingsare all global settings that can be made in the Menu
