Why did my Apple Watch stop recording my VO2 level?

Is your Apple Watch on the latest watchOS/? If not, update the watch

Have you tried to restart the phone and watch and after that record a 20 minutes walk or run?

see If the VO2 max Is back after that

if it is not back:

check the Motion & Fitness privacy settings. On your phone go to Settings > Privacy >Motion & Fitness, make sure Fitness Tracking and Health is turned on. On your phone, in the Watch App, to to My Watch > Privacy and make sure Fitness Tracking and Heart Rate is turned on.

Also see this link for further reference

Set up Cardio Fitness Levels – Apple Support (UK)

I also had this issue with the 5, and I just upgraded to the 8 which didn’t fix the issue. Data source is blank under Cardio Fitness (and now wrist temperature as well on the new watch). Apple technician said the only way vo2 records is is you are 3x resting heart rate sustained for 20min with little variation in that rate, and on less than 5% graded surface for at least 3 workouts before it records. So it seems like it’s only available for the super athletic workouts. Which seems pointless for those who are trying to improve their health.

Kinda wonder what the answer is going to be for not recording my temperature now.