This link should help
Found via >> Support Services | Tableau Software
If you haven't been contacted by your sales manager, I would suggest emailing - email@example.com
Mark's answer is spot on! As an addition to this, occasionally the maintenance has been renewed by your accounting department, but the maintenance date needs to be refreshed on your machine (it's not automatic upon payment).
From Desktop, go to: Help > Manage Product Keys > click Refresh. If the maintenance has been renewed, this will bring in the new end date. If not - reach out to the above resources Mark noted.