umm.... if your watch successfully recorded it then yeah it should be in the activity app on your iPhone (among other places). If it's not in the activity app then it was not recorded successfully and no way to recover.
Could have not recorded due to failure to initiate the workout, perhaps a bug/crash during workout, or perhaps if you someone discarded the workout upon completion (I don't think I've ever discarded a workout before so I'm not sure if any of that data is recoverable).
If the workout isn't in the activity app check the health app (also on the iPhone) and check your HR during that time to see if you had a continuous HR.