How do I recover my workout watch face on my Apple Watch?

To change what you want on your watch face- hold down your screen until the watch face zooms out, you should then see all of your installed watch faces, at the bottom- click "Edit" on the watch face you want to edit. Then scroll left or right until you see "Complications" at the top of your screen. You will be able to add or take off what you want.

Did you make sure that the Workout App is still installed on your watch?

If not, try to download it again, using the AppStore on the Watch.

After that it should be available again as a complication on your watch face.

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copied from: Redownload apps and games from Apple - Apple Support