Missing exercise time in the Fitness app

Has it ever tracked minutes?

or Standing?

Check Watch app, Privacy, is fitness tracking turned on?

In Watch app, Health, are your details complete?

if all seems ok...

First force restart the Watch 

i.e. press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo. 

 Then, although it may seem irrelevant, while it is restarting Force Restart the paired iphone   Force restart iPhone – Apple Support (UK) 

Test it for a while. If it is still misbehaving... Unpair and set up again

 Unpair and erase your Apple Watch – Apple Support (UK) 

and then set up without restoring the backup

Set up your Apple Watch – Apple Support (UK) 

You may also want to make sure the software on the watch and iphone  are both up to date to the same level. Updates are in pairs. 

Update your iPhone or iPad - Apple Support

Update your Apple Watch - Apple Support

Green ring?

Did you start an outdoor walk workout?