To calculate the capital loss of an investment, you need to know the “adjusted cost base”. The Canada Revenue Agency (CRA) generally defines adjusted cost base (ACB) as “the cost of a property plus any expenses to acquire it, such as commissions and legal fees”.
See this previous question for more information about calculating adjusted cost base.
You will need to know how much you paid for your holdings (whether all at once or overtime), how much you sold the securities for and any associated transaction costs to buy and sell your holdings. If you do not have this information, you may wish to contact the investment firm where your securities were held. The Ontario Securities Commission is not able to provide advice. Speak to a registered financial advisor or tax professional to help you calculate ACB.