There a number of causes for this, most notably if the User Profile Synchronization service isn't running (or has had a problem) or, as I have found, is often due to services.
The symption is shown when opening a My Site:
Still no luck? Seems every now and then SharePoint likes to stop a service, specifically the ForeFront Identity Manager service. Check this through Administrative Tools > Services:
Notice it is NOT running but IS set to Automatic - go figure. Simply click Start the service. Wait a few minutes and access the My Site again and all should be good!