A new device

Your address book is encrypted on your devices; the server can’t read it. So on a new device it stays closed after you sign in, until another of your devices or your recovery key opens it.

From another of your devices

Sign in on the new device. “My computers” shows “Your address book is encrypted” and this device’s code.
On a device where you are signed in already, “New device asks for access to your address book” appears with a code.
If the codes on both screens match, press “Let in”. The book opens on the new device.

With the recovery key

If no other device is at hand, press “Enter recovery key” and type the 26 characters you saved when you created your account. Case, spaces and dashes don’t matter.

If the device isn’t yours

Press “Deny”. In the bar “New sign-in to your account from … Was this you?” choose “It wasn’t me”: the device is disconnected from your account. Then change your password.

With neither a device nor the key

Then the book can’t be opened: only you have the key, we don’t keep it and can’t restore it. That is how end-to-end encryption works. Keep the key somewhere safe; a new one can be created under “Account” on a device where the book is open.