Make a cell mandatory so that user can't navigate away from sheet until its populated

Hi

I want to make certain cells in a column mandatory based on the value of the cell in the previous column so that the user can't navigate away from the sheet until they have entered something into the mandatory cell.

e.g. amount exceeds guidelines so they have to enter an explanation for the exception.

Any ideas?

Thanks

This what I've got so far:
Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)
With Worksheets("D - GOSCE - Protected Data")
If .Range("AR6").Value = "TRUE" And .Range("AS6").Value = "" Then
MsgBox "Mandatory cell not filled. Please add exception explanation before proceeding"
Cancel = True
End If
End With
End Sub

So basically if the condition is TRUE it prevents them from saving the file. It's working on this one cell, but how do I amend the code to cover a whole range of cells (1731 to be precise!)

Thanks