Sonos Playbar has dropped off my system

I have a Sonos Playbar which has dropped off my system i.e. in my Sonos App, on the “Systems Settings” page, there is a subsection entitled “Your Settings”, which for the Playbar in question, the App says it is “Not Connected”, whereas all the other Sonos devices are listed by what they are eg Playbar, Arc etc 

Firstly, I don’t understand why this Playbar has dropped off as I have 4 other Playbars, which are all still connected.

Secondly, and most importantly, how do I go about reconnecting a Playbar which has dropped off my system/App?

I’m asking for help as I had an AV Company install my entire AV system, which Sonos just forms one part of, and this is all out of my skill set. 
 

Thanks in advance.