Arlo cameras went offline a week after I left the house.

[FrankH44]

Before departing on holidays, I inserted new batteries in five Arlo cameras. Synched them. All were working perfectly, at least for about two weeks. As of yesterday, I have the message that the cameras are offline! No, no none can enter the premises to manually handle the cameras. Although this is the first instance, I am not pleased, and feel very helpless.

[jguerdat]

Try the app and a browser - same result?  AT this point, all you can try is to restart the base if you can get to the Restart button in Settings, My Devices, your base.

It's possible that you had a brief power outage that caused the base to go into an unknown state.  A UPS/battery backup for your modem, router and base helps fly through those.  I use a 750VA unit with the expectation of several hours of run time.