If your board is paused in a view-only mode even though you haven't hit the limit of three active boards on the Free plan, this article will help you 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 in a view-only mode 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 and available for the board owner and explicitly invited users for viewing only. When hovering over the board you'll see the message: Free plan users can collaborate on 3 active team boards. Private boards are locked.
Inactive board on the dashboard
To start working on the inactive board again, you should share it with your team.
Sharing a private board with the team
When you move a private board to a free team
Once a private board is moved to a free team, it gets locked in a view-only mode unless shared with the team.