Setting the field's Visible setting to NO means that it cannot get the focus, and therefore does not need to have data entered. No Problem.
When the control is set to Visible = Yes (presumably by the After Update event of the Combo Box?) also set the focus to the control. Then in the control's Before Update event, test to see if it contains any data. If Yes, do nothing and proceed to the next control (or whatever you need to do). If No, pop up a message box to advise that the user must enter data in this field, and cancel the move to the next control (Cancel = true). This way the users won't be able to do anything until they enter data in the field.
Something like:
' Calls the MANDATORYFIELD function.
' Shows a messagebox advising that this field must contain data if
' the control field is set to YES.
If IsNull(AHDetails) Then
Call MandatoryField(Me.ActiveControl.Controls(0))
Cancel = True
AHDetails.SetFocus
End If
Public Function MandatoryField(ctl As Control)
Dim strField As Variant
strField = MsgBox(ctl.Caption & "@This field MUST have information entered into it.@" & _
"Enter the required information or press the [ESCAPE] key Twice to delete " & _
"all data on this screen and start again.", vbOKOnly + vbInformation, _
"Required Information Missing!!"

End Function
HTH
Lightning