Hi
To add Workout data manually to the Health app:
On your iPhone, in the Health app (red heart on a white background), go to: Health Data (tab) > Activity > Workouts > tap the "+" sign (upper-right) > enter data as desired.
To just add Active Calories data manually to the Health app:
On your iPhone, in the Health app, go to: Health Data (tab) > Activity > Active Energy > tap the "+" sign (upper-right) > enter data as desired.
Alternatively, if you make regular encrypted backups of your iPhone (such as automatic nightly backups via iCloud), then - depending on which day's data has been deleted and whether or not restoring from backup would cause a further gap in more recent data - you may wish to consider restoring your iPhone from an encrypted backup and re-pairing your Apple Watch.
Keep in mind that data since the date of the backup would be lost. Note also that unencrypted backups (backups made via iTunes without enabling the encryption option) do not include Health or Activity data: