You'll need code in two form events.
In the Form's Current event:
If Me.Status = "Closed" Then
Me.AllowEdits = False
Else
Me.AllowEdits = True
End If
In the Form's BeforeUpdate event:
If Me.Status = "Closed" Then
If Not IsDate(Me.ClosedDate) Then
Cancel = True
Me.ClosedDate.SetFocus
Msgbox "Please enter date closed.",vbOKOnly
Exit Sub
End If
End If