The first two places that might be worth a sanity check:
(1) By any chance were Subscriptions temporarily disabled for some reason? That is, in Tableau Server Configuration there's the Alerts and Subscriptions tab and there is a second to "enable Subscriptions" and directly below that to "enable Health email alerts". Is the one box still checked?
(2) There's the "Background Tasks for Non Extracts" Server Admin view. Does it show Subscriptions trying to run? (but possibly they're hitting errors?)