Suppose that younger you played tennis. Lack of availability: you have stored your racquet in an attic. Without maintenance, it has deteriorated. Not uniformly. After years, the leather handle first shrunk. Then the hoses of the rope dried and finally let go. The fiber frame did not change. You suddenly decide to play tennis again! You know there’s a racket in the attic. When you find it, you can see that it is unusable. Your intention pushes you to renew the rope. And the handle. The racquet becomes functional again.
The memories are so. Structured with elements more or less resistant to time, depending on whether they are used by other memories or not. When an association is no longer maintained, its neurological path is diminished. But it is not all the architecture of memory that has disappeared. If the context that created it reappears (the equivalent of your intention to replay at tennis), it will redo the path of the complete memory. The associations are reforming. The word you missed the moment before, or the lost memory, reappear.
Remembrance is not monolithic information, but a pyramidal mental scheme, of which we easily lose the upper floors, rarely the foundations. The foundations recreate the floors if the data to be processed reproduce the conditions in which the memory was formed.