You will need to know "the basis", what you originally paid. You can go online for historical information on stocks. (Follow this link for the historical information on HP: http://h30261.www3.hp.com/stock/historical-lookup.aspx) It is acceptable to take the high and low price of a stock and average them to arrive at a value.
Having some "best efforts" cost basis figure is better than not having anything at all.
The following TurboTax information may be helpful in finding your cost basis.