A person's injury status can only be managed through the Events module.
As an example, if a person is set to "Absent From Work" when an injury is entered, this will be reflected in their record in the People module, like this:
As you can see, the Event ID is also shown in brackets next to the injury status. This is so you can find the Event that they were involved in.
As good practice, the person should be set back to 'Fully Fit' before the Event is completed and closed out. If the Event is deleted (i.e. after testing or demonstrating), or the person involved in the event is changed, you can no longer manage the person's injury status through it.
To get around this, you will need to add another injury into the Events module for the person. The key part is ensuring that you set the Current Medical Condition to 'Fully Fit':
Save the Event after filling out the mandatory fields.
After confirming that the person's injury status is now 'Fully Fit' you can delete the Event.
Note: adding an injury may trigger notifications - it is recommended that you make clear in the description that this is not a real Event and should be ignored.