Your boards remain accessible to view after your subscription or trial expires, but from that point it is not possible to add posts to boards and it is not possible to create new boards.
Regaining full editing access to boards housed within an expired trial or subscription can be done through upgrading to a paid business plan. Upgrading can be done in the Plan tab, which can be reached through clicking Upgrade at the top of your screen or through first clicking Admin Panel and then the Plan tab.