OK. Try changing:
Dim startDate As Date
to:
Dim startDate As Long

and changing;
startDate = CDate(InputBox("Please enter  Start Date: ", "STARTDATE"))
to:
startDate = DateValue(InputBox("Please enter start  Date:", STARTDATE"))

I'll take a guess that vlookup  doesn't like to handle the Date
variant; from excel help:
"Lookup_value can be a value, a reference, or a text string"

and include some more code to check for a valid date  input by the user.