This means that you need to re-enter your Outlook details into Ezekia again. It can happen if you have changed your Outlook password, if we have added new features to the Microsoft integration, or if Microsoft mandates it for some reason.
To re-authenticate Outlook sync, navigate to the settings page (via the 3-line icon in the top right corner), scroll down a little and click the icon shown. You will be prompted for your email address and email password. Once these are entered, synchronisation should work again.
If it fails to authenticate, it may be because it is defaulting to a personal email address. The easiest way to resolve this is to switch to an ‘in private’ browser and authenticate again there. Once it has worked, you can go back to your usual browser.