If these were inherited securities, you'd get a step up in basis. The step up means that the basis of the securities is increased to their fair market value on date of the deceased owner's death. You'll need to find the value of these securities on the date of death to enter that as the cost basis and calculate your gain or loss.
If these were not inherited securities, you'll need to find out what they were worth or purchased for at the time of original purchase.
You can use resources such as online cost basis estimators, old account statements, or online account or transaction history from your broker to recreate your cost basis. Just make sure that you keep records of whatever you use to calculate your basis.
