If your staff member does not normally come into work this day and you have 'hide non-working staff' enabled on your system please follow the steps here to add them back into the calendar.
Firstly, click on 'Staff View' on the calendar. It's on the top left!
This will show you a list of your staff names. Select the staff member you would like to add back in.
This will show you a weekly schedule for this stylist only.
Click on the day with 'time off' added all day. Click the time off block and select delete!
This will show the staff column again on your 'Day view'.