The arpeggiator, sequencer, LFOs (when set to clock sync) and delay FX (when set to clock sync) are driven by either the internal clock or an external MIDI or Sync-In clock source.
If your ARGON8's 'Clock Source' setting value is set to 'MIDI' or 'Sync-In' when there is no connected MIDI / Sync source or no MIDI-In / Sync-In clock being received, this will result in the above elements not being able to work. If the arpeggiator is not able to run, this will also result in the keyboard not working.
To fix this, go to the 'Settings' screen page and set the 'Clock Source' setting to 'Auto'. This will allow ARGON8 to automatically use the internal clock if no external clock source is detected. If multiple external clock sources are detected when in 'Auto' mode, ARGON8 will prioritise Sync-In over MIDI.