I have a substantial gain in Box E Long term sales with cost basis NOT reported to the IRS,why is gain reported but not cost basis? the amount made me now owe on taxes.

You must have paid something for the stock--this is where you may have to do a little investigative work.

If you don't record cost basis, you have to pay taxes on the whole sale.

You may be able to do an internet search for the date acquired and the stock name.