You will need to file an amended return. You will need to try and recreate the most accurate records possible. I suggest you start with the IRS and request a Wage and Income transcript. You can go through your statements, spreadsheets, whatever details you have.
The IRS believes you should have an excel spreadsheet where you should track investments basis, date of purchase, etc. on your own. I think a great solution is to use a service, like our investor center. See TurboTax Investor Center
See How do I amend my federal tax return for a prior year?
I want to urge you to create a financial notebook that is kept separate from your tax return. Keep it safe and each year, add your year-end statements from all your financial accounts plus a copy of your W2’s, your carryover information, and proof of your basis in your various investments. You must keep tax records from the time you purchase until sold/ loss used plus 3 years. It is very easy to lose track of disallowed losses, carryforwards, and basis. This can be a digital or paper notebook.
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