I wouldn't be surprised if there are inaccessible, partly corrupted memories encoded in the hippocampus. I suspect most of them cannot be prompted by a therapist though, and likely there is no practical way to recover them.
I think it's all a matter of finding a trigger (or reference) to grab the memory. A therapist talking to you almost certainly wouldn't achieve that, but walking down the street and smelling an odd smell might.
That makes sense considering that human memory is strongly based on associations. Activating nearby memories can bring things back.
If you hear the first tones or words of a song you're much more likely to be able to tell the lyrics that follow compared to being asked to say those lyrics based on the title.
