What happens in the brain that allows access to a previous unavailable memory? What happens in the brain that allows you access to a memory previous unavailable?

For example I forgot the name of someone I've known for years. Two weeks later, without notice, "Diana" popped up. 

Why was it not accessible when I found I couldn't remember?

What was the chemistry that occurred that allowed it to happen later?

(Secondarily, is this the same breakdown in dementia and alzheimer's? That is, a breakdown of the 'search memory' mechanism? Or is that the memories themselves are 'losing coherence.')
These kinds of research questions are interesting, but notoriously hard to investigate empirically, because:


You need human subjects that are able to provide complex feedback, i.e., you can't use invasive, long-term recordings in animals;
You need subjects to undergo long-term experiments, as your example indicates in the order of weeks, for one single measurement;
You need subjects that experience this regularly and frequently;
You need a way of faithfully time-stamping the exact moment of recall and somehow establish a procedure to grasp what's happening inside the brain against a variable, uncontrolled background.


In essence, it's next to impossible to find out.

There is, however, a large body of literature around memory recall in general, including detailed imaging studies, e.g., Heckers et al. (1999), Hall et al. (2001), Liu et al. (2012) and I'm sure there will be much overlap between spontaneous and deliberate memory recall.    

References
- Heckers et al. Arch Gen Psychiatry (1999); 56(12): 1117-23)
- Hall et al. J Neurosci (2001); 21(6): 2186-93
- Liu et al. Nature (2012); 484: 381–5