If the individual tested the internet connectivity with a phone, it may have switched to Cellular. Are you sure they were on the actual network at the time?
Also, I have experienced this when my network was experiencing issues as @speadie indicated. Do you have a WCO Camera? Asking because the base station would normally be hardwired to the Router and the camera would connect to the base station via a secure channel. if that is also having an issue, I would look at the router / Modem or simply reboot it and let it sync up again.
Here is why: I had an issue with my Router, years past. The issue was as follows, everything looked as if it was up some of my device was able to get out on the internet, but it was very inconsistent. once I put a load on it, things started to really become flaky. turned out to be a Network Card at one of the accee points outside of my house. they simply moved me to another port at the main box, and everything came up and was stabilized.
If you want to try something a little simpler, you could clear your cache from within the app and log out of the app. Then, if you are on an Android device, you could Force Stop the App and also clear the cache form phone side. Then log back into the app and let it come up. then click on a camera and see if it wakes up.
do you haveplugs ot bulbs as well? If so, do those work?