Lack of alerts in Sentinel

RayO​ I've been scratching my head over this same exact issue over the last 4 days. Your workspace is probably in a disconnected state on the defender side like mine was.

My fix was to go to Microsoft Defender (security.microsoft.com), go to System>Settings>Microsoft Sentinel. From here you should see all the workspaces you have configured (make sure you have access to the resources for them to appear in this list). Select the one you need and hit Connect, give it about 15-30 minutes and you should see the data start flowing in and the data type should be lit green. Sucks that there isn't any useful information that covers this specific issue but I'm hoping this works for you!