If you're unable to open your board and see the message The private board is locked, this article will help you to understand why it's happened and how you can unlock the board to continue working on it.
When you downgrade to the Free plan
As soon as you downgrade your account and change the subscription type to the Free plan, the boards might get locked for two reasons:
1) The account limits were exceeded. The Free plan allows creating an unlimited number of boards but only 3 most recently created ones are active.
2) The board becomes inactive because it was created as private when on the paid plan. Since board privacy is not supported on the Free plan, the board gets locked.
Inactive board on the Dashboard
To start working on the inactive board again, you should open the board and click Share with your team in the popup window or upgrade your account: