Database value doesn't display in wpf combobox

I've messed something up but cannot figure out what.  I have a datagrid that used to display a value coming from the database just fine.  Then I changed the cell on the datagrid to a combobox.  The combo drops down with the correct items but now the combo does not display the value coming from the database.

Xaml:

<DataGridComboBoxColumn x:Name="cboDentistAppointmentTimes" Header="Appointment Time" TextBinding="{Binding DentistAppointmentTime}" />

Code to bind combo:

        Dim AppointmentTimes As New List(Of String)() From { _"8:00", _"9:00", _"10:30" _
        }

        cboDentistAppointmentTimes.ItemsSource = AppointmentTimes

When the datagrid is displayed, no values from the database are displayed.  The cell is blank.  How do I get that value back?

Thanks.

Kris Hood