If you have a 1099-B with proceeds then you must report it. The cost basis for the long term transactions must be calculated by your own records. The brokerage either does not have the correct records or the history goes before the requirement for them to track it. The cost and not the value is what you are allowed to use to offset any proceeds received for your stock.
There are many sites that could help you with that if you know your purchase dates and do not have all of your records.
Items of Interest to calculate cost basis:
- Number of shares and cost purchased on each date
- Dividends reinvestment (cost basis) dates and shares purchased
- Stock Splits dates and number of shares before and after the split
- Nontaxable distributions or returns of capital (reduces cost basis)
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