Workout App - Apple Watch S8

Tap the Digital Crown and then see the 'Start a workout' section in:

Stay fit with Apple Watch - Apple Support

(Look for the green circle that looks like a man running inside of it.)

Also see:

Use the Workout app on your Apple Watch - Apple Support

If you still can't find the Workout app, the first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:

How to restart your Apple Watch - Apple Support

Restart your iPhone - Apple Support

If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.

Unpair and erase your Apple Watch - Apple Support

Set up your Apple Watch - Apple Support

As a last resort, follow the above procedure for unpairing and pairing the watch, but do not restore the backup. Set the watch up as new instead. There should be no loss of Fitness data or Awards since those are stored on the paired iPhone, not the watch.

Do find the workout app as suggested.

You can also use Siri on your watch to start your workout, a feature I use regularly. For instance, tell Siri on your watch...

Start Workout Tennis

Start Workout Outdoor Walk

Start Workout Indoor Walk

etc.