Disconnected WhatsApp, can't reconnect!
I had the same issue and have now found the answer! The answer is that the version of Whatsapp which you used for restoring chats is not automatically reconnecting nor updating from the play store.

First of all, make sure you have made an internal backup of your chats, easy because backup to Google Drive no longer works.

Now uninstall Whatsapp - no worries, your content is backed up on the device.

Go to the Play Store and select Whatsapp. Install it.

Whatsapp will go through all the motions and find the local backup. It will ask to import it - allow that.

As a last step, the new Whatsapp will ask to connect to Google Drive for periodical backups - just answer what you prefere here.

Go have a look in Google Drive and see Whatsapp now appears once again in the list of connected apps.

All done!