Hello @WilliamM ,
Please following steps:
1.Sign in SharePoint admin center as a Global Administrator to check if the site exists
2.If the site exists, please check if the site is locked. If the site is locked, there will be an error that "This site can’t be reached".
3.You can run the below powershell command as an admin to unlock site
#Connect the SharePoint online Admin center
Connect-SPOService -URL https://tenant-admin.sharepoint.com
#Get the SiteCollection URL
$SiteURL = "https://tenant.sharepoint.com/sites/du"
#Unlock the SharePoint Site
Set-SPOSite -Identity $siteURL -Lockstate “Unlock”
#Lock the SharePoint Site
#Set-SPOSite -Identity $siteURL -Lockstate “NoAccess”
Thanks,
Echo Du
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