Filing taxes on stocks
You need to submit an updated return. The problem is that once three years have passed you can't update the return to get any kind of refund, but if they are going after you for the sale price of the stocks, not knowing the cost, your goal is to show them there was no gain, and in fact you'd have had the loss if you were aware of the account. This is less than ten years back, so the broker should be able to give you the statements pretty easily.