sherif28,
I would think you would be entitled to half the money. Have you spoken to your ex. I am not sure of your situation but if you settled your divorce by means of a separation agreement, one of the conditions of a separation agreement to be valid in Ontario is that FULL financial disclosure occur between the parties. If this does not occur the separation agreement could be varied or possible voided and net equalization could be re-calculated.
I would think the Judge would be somewhat sympathetic in your case as FULL financial disclosure is to occur when it comes to net equalization payments.
You would have your ex's previous sworn financial statements or could be found in the continuing record.
What I would do is get a statement of this joint account going back to day one. To open a joint account, they the bank would have your signature on file.
Get records of deposits etc, who made them. If it was a large sum of money it most likely the deposit was done by cheque, perhaps from a stock broker
I think you will have no problems from the bank on disclosure, After all the account is listed as a joint account.