There are multiple possibilities

• the Sync'Em Engine is paused because:
• it was paused via the menu extra (i.e., the "Pause Sync'Em Engine" entry has a check-mark to its left — select it again to un-pause the engine)
• the Sync'Em preferences panel is open — that automatically pauses the engine so settings can be changed without the danger of having an undesired sync when part of the way through a setup change
• the Sync'Em Engine is no longer running (the menu selection above the "Pause Sync'Em Engine" entry provides a status that may give a clue) because:
• the Sync'Em Engine was manually stopped — e.g., via the Sync'Em preferences panel
• the Trial license has expired or Sync'Em has otherwise become unlicensed — you can check the licensing status via the "About" panel in the Sync'Em preferences
• the Sync'Em Engine crashed — to restart the Sync'Em Engine:
• open the Sync'Em preferences and click on the "Preferences" tab ("User Prefs" in older versions)
• if it's available, press the "Stop Sync'Em Engine" button (admin credentials required)
• press the "Start Sync'Em Engine" button (admin credentials required)
• please help us find the issue

Also see the Knowledgebase article about the Sync'Em Engine in a "hung" condition.