Calendar app disappeared from Apple Watch (still on iPhone)

My calendar app disappeared from my Watch (in the middle of the night). It doesn't show up in Complications or in installed apps -- but it's still on the iPhone (and working fine there).

The Watch still shows the date in the Complication spot, but when you touch it to link to the app, it says, "Restore "Calendar"? You followed a link that requires the app "Calendar", which is no longer on your Apple Watch. You can restore it from the App Store." However, since it's still on the iPhone, when you go to the store, it just says, "Open."

The Watch calendar app has been working fine since I got the phone a few weeks ago -- until last night.