Can you perhaps just briefly list your Sonos devices. State if they are wired/unwired to the LAN and if you have ever switched off their WiFi adapter?

Also what router are you using?

Any other wireless access points, power-line adapters, switches (managed/unmanaged) etc?

I have 1 arc with 2 play:1 and 1 sub. In other rooms, 1 One, 1 Roam and 1 One SL. 
They are all wireless, connected to a netgear orbi. Everything was working perfectly fine until I unplugged the surround speakers. They disappeared from my system so I tried to add them again. That is when everything went wrong. 

See if this suggestion helps to sort things…

Take one of your standalone speakers (such as the One or One SL) and cable it to the Orbi primary hub/router - if you have another non-Orbi WiFi router besides, such as one supplied by your ISP, then just ensure it’s WiFi is switched off.

Your mobile controller needs to connected to the Orbi WiFi - just check that is the case.

Note - only the one Sonos device should be wired.

Lets just see what happens when you try these things.

Thank you very much for your help. 
This morning, everything is working perfectly fine again, without any action from me. 
I’ll keep your advise in mind for the future, in case it’s happening again. 

It sounds like IP address confusion in your router.  Most routers are set to refresh IP addresses overnight, so this is what probably cured the problem.  You would need to reboot the router AND all the Sonos devices as a minimum to avoid waiting for the next refresh.

Yu might prevent it from happening again by assigning reserved IP addresses to your Sonos devices.  But if it is working now I’d leave well alone.