I used to say that I had programmer's amnesia, which is that, if I didn't flex my programming muscles often enough, I would completely forget how to do it.
After reading The Talent Code, I'm now of the opinion that it was a loss of myelin.
Here's my fuzzy explanation: when you do something, you're firing a nerve impulse. For every time you fire that thingy, myelin starts to wrap around it and insulate it, which makes you more efficient at firing that thingamabob in the future. However, myelin is a living whatchamacallit, so it can deteriorate if you stop using it.
So basically, it's probably not full-blown amnesia. It's more so the fact that the myelin wasted away and then it feels like amnesia.
So the takeaway point is something that I already had assumed. Programming is like any other skill, you have to frickin' practice it regularly or you will cease to be as awesome as you once were.
Or in my case, as mediocre as you once were.