Cost or Basis Tax Question

I am completing my taxes and have run into a challenge. I bought shares of my employer’s company stock a few years back and have held on to it for many years. The company was recently bought by another company and the stock was then converted to the new company stock. I have now received a form 1099-B Proceeds from Broker and Barter Exchange Transactions with date of sale\exchange and stock value, federal income tax withheld. In researching the cost or basis calculation it is the value when bought and value when sold percentage I believe? I did receive an account summary from the trust company but it outlines annual sales details from 2005 until the sale of the company to new company. I bought the shares previous to 2005 and have not sold anything since purchase. Confused on how to calculate the cost or basis at this point. Any help or direction would be greatly appreciated.