This is a tough question to answer because our intuitions mislead us.
It’s tempting to imagine that we have lots of neurons, and lots of memories, and so perhaps each neuron represents a different memory, a little like the 1s and 0s on a hard disk.
But in fact, your brain distributes memories over a network of inter-connected neurons (brain cells). Each memory involves multiple neurons, and each neuron is involved in multiple memories.
Here’s a way to picture it. Let’s imagine we belong to a troupe of actors performing a play.
I know my lines, and you know yours – but we don’t know one another’s.
So, if you’re in the middle of a speech, I’ll recognize the end when you get to it, and I’ll start my lines at the right moment. Likewise, when I’m done with my lines, the next person recognizes their cue, and starts their own lines. We each know our own lines, and just enough of the end of the previous lines to stitch it all together.
So, when we get the whole cast together, we know the play perfectly – as an ensemble – even though no single actor knows more than a fraction of the entire play. Our knowledge of the play is distributed over the whole cast, with no single actor knowing the whole thing.
This is a little like how the brain stores a memory over an ensemble of neurons. Each neuron’s activity supports the others, and so the memory is distributed over the ensemble.
Let’s say one of the actors is sick. Probably the fluency of the performance will suffer – we’ll stutter when we get to his lines, but we’ll probably be able to reconstruct most of his bits, and improvise a little, so the play won’t grind to a complete halt. Of course, the more actors we lose, the more the play will degrade, until eventually it’s not even recognizable as the original play.
This is pretty much how the brain works. Even if a bunch of neurons die after a heavy night of drinking and concussions, you won’t lose entire memories wholesale – it’s just that the memories which involved them will be slightly weakened.