make a quick test file
see what this is going to do. if its what you want, then try it in your original

– Right Click on your sheet1 tab and choose View code (or just hit Alt F11)
– cut and paste this

[vb]

Sub SentenceCase()
For Each cell In Selection.Cells
s = cell.Value
Start = True
For i = 1 To Len(s)
ch = Mid(s, i, 1)
Select Case ch
Case "."
Start = True
Case "?"
Start = True
Case "a" To "z"
If Start Then ch = UCase(ch): Start = False
Case "A" To "Z"
If Start Then Start = False Else ch = LCase(ch)
End Select
Mid(s, i, 1) = ch
Next
cell.Value = s
Next
End Sub

[/vb]

– then highlight your column (or whatever you want to change)
– then run the macro
– click tools
– click macro
– choose sentance case and run