1. You can report your card lost 24 hours a day at the BearPass Card Account Management application. This will temporally freeze your card so it cannot be used.
2. You can also report your card lost or stolen during normal business hours at the BearPass Card Office in the lower level of Plaster Student Union. The BearPass Card Office will, upon payment of a $25 replacement fee, make you a new BearPass Card.

More info on the BearPass site here.