Office 365 Groups retention policies are very helpful to reduce orphaned groups in an organization's tenant. Kindly, the Office 365 system sends out notifications to inform the group owner(s) to renew a group - or that a group will be deleted. What, if you oversee such a notification? See here how to restore an Office 365 group - thanks soft delete!
So, in my case, I oversaw the notifications and saw that my "projects" group was no longer accessible. I got a HTTP 403-Forbidden message when opening the SharePoint site of that group.
Before an Office365 group is deleted...
...the owner gets an email with the option to check the group and to renew the group - so that it's not being deleted, as follows.
Of course, I forgot the renewal...
When a group is already deleted...
In my case, I found an email informing about the group deletion later:
The good part is that the deleted Office 365 group can be restored within 30 days - and the email informs about that. Simply click the "Restore group" button and follow the steps.
Do it as Admin
If you are not the owner (or simply don't find the email), you can use the Exchange Admin for restoring an Office 365 group. Just to mention, it's not possible to restore an Office 365 group in the Office 365 portal or in the (new) SharePoint Portal, that's an Exchange task. So, open the Exchange Admin Center, go to recipients and to groups as shown here:
On the right side, you find the "Click here to restore" link. Then, confirm the undelete process.
The group will be undeleted and will be available usually after some minutes (but it can take some hours as well).
The Restore feature is pretty cool, see New feature: Recover deleted Office 365 Group via PowerShell.
Manage with PowerShell
You can do it with PowerShell as well with the AzureADPreview module and Get-AzureADMSDeletedGroup and Restore-AzureADMSDeletedDirectoryObject -Id <objectId>, see Restore a deleted Office 365 Group. If you really want to hard-delete a group (Purging the group removes the group and its data forever!), use Remove-AzureADMSDeletedDirectoryObject -Id <objectId>.
If the soft delete period of 30 days is exceeded, there is no out-of-the-box restore-option (hard-delete). For such a case, you need third party tools or other solutions.