I just recently bought a prepaid Koodo Nokia phone with the free $20 activation bonus and then i bought $35 dollar Koodo Voucher. Now my when I check the phone, it is currently saying expired plan and such.
Log onto Self Serve for Prepaid and see what it says about your Base Plan (when it expires, etc.) If it says that it has expired/doesn't exist, choose a base plan and pay the top up that amount using your $35 voucher.
If you have done this and it says that it's expired, contact Koodo Customer Service.