When a connected account loses access, for example after you change your iCloud password or revoke access at the provider, the Keeper.sh dashboard now says so and takes you straight to reconnecting it.
A Reconnect notice in the sidebar lists every account that needs attention, with each provider's icon, and the calendar view shows the same notice above the grid. Choosing an account opens its reconnect screen.
Once you reconnect an account, Keeper.sh syncs it again.
Copied events have disappeared covers what happens to copies when an account loses access.