I don’t believe the connection between the outputs in HALion Sonic SE and the Mixer is bi-directional, so you can mute each side independently of the other. In HSSE’s window you can mute an output and Dorico’s Mixer will not show that the output is muted; it will simply receive no signal. Likewise, if you mute an output in Dorico, HSSE’s window will not show that the output is muted, but no signal will make it to the output because it is muted by Dorico.
You can solo a selected staff with the shortcut Alt+S in Dorico, which is maybe what you did by accident? If you run into this problem again, you can use use Play > Deactivate All Solo States and Deactivate All Mute States to reset the Mixer back to playing everything again.