If your card was registered, you should call 1.877.NOW.VENTRA and report the card lost. (To do so, please have your card number and four-digit PIN ready.) Ventra's customer service representative will deactivate the card. Any pass or transit value in your account will be protected. You will be sent a new card and charged a $5.00 card replacement fee.
If your card was not registered, any pass or transit value is not recoverable. You can get a new Ventra Card by ordering it online, or going to a Ventra vending machine or Ventra retailer.