Apple Watch SE stopped detecting movement

Have you tried to calibrate it to see if that works?

1. On your iPhone, open the Settings app.
2. Tap Privacy & Security > Location Services. 
3. Make sure that Location Services is turned on.
4. Scroll down, then tap System Services.
5. Make sure that Motion Calibration & Distance is turned on.

1. While wearing your Apple Watch, go to a flat, open outdoor area that has good GPS reception and clear skies.
2. If you have Apple Watch Series 2 or later, you just need your Apple Watch.* If you have Apple Watch Series 1 or earlier, take your iPhone for GPS. Hold your iPhone in your hand, or wear it on an armband or waistband.
3. Open the Workout app, then tap Outdoor Walk or Outdoor Run. To set a goal before starting, tap the More button.
4. Walk or run at your normal pace for about 20 minutes.