Your email sync can become unsynced if something chnages on your email provider's end. For example, if you've changed or updated your gmail/ outlook password - this would cause your email to stop syncing in Loxo.
To resolve simply click on the resync button.
Navigate to Settings under the gear icon in the upper right-hand corner.
Click on Synced Emails on the left-hand side.
Check to see if your email is synced. Simply hover over the email you want to check. A correctly Sync'ed email account will show "running" as the sync state.
If yours says Invalid, you'll need to resync.
Click on the resync button