I was queuing for a ticket at Clapham Junction when it happened. The train was leaving any minute from a platform at the other end of the station, so I was tense. To add to my woes, the person in front of me using the machine was one of those professional ditherers the Sod's Law Corporation apparently employs to arrive in your life at the most infuriating moments.

As time drained away, he gawped at the screen like a medieval serf trying to comprehend helicopter controls, confounded by one simple question after another - questions such as where he was going, and how many of him there were. I ground my teeth to chalkdust as his hand hovered over the touch screen, afraid to choose, like a man deciding whether to stroke a sleeping wolf.

Finally the prick was done, and once I had waited for him to collect his tickets and his bloody receipt, it was my turn. Having no change, I opted to pay by card. But just as my hand moved toward the keypad to enter my pin, a voice in my head whispered: "You don't know what it is." And it was right. I didn't. I scanned my head, but nope: my pin had vanished. It had gone.

I tried inputting something that seemed about right. "INCORRECT PIN," said the screen. I slowed my breathing to clear my head. Rested my hand on the keypad a second time. Tried to fall back on muscle memory. Performed a finger dance. "INCORRECT PIN."

I became aware of the snaking, sighing queue behind me. Now I was the ditherer. A third bum guess would swallow the card, so I snapped it back into my wallet, turned on my heel and walked off, past the eyes of the queue, trying vainly to look as though not buying a ticket had been my plan all along, and everything was going smoothly, thanks for asking. Annoyed, I went outside and hailed a taxi.

As I sat in the back, I examined the contents of my head. The number had to be in there somewhere. After all, I've only got one card. One pin to remember. And I use it all the time, every day; in supermarkets, cafes, cashpoints, stations ... everywhere. I realised that I'd better remember it soon or I wouldn't be able to function in modern society. Yet the harder I thought, the more elusive the number became. The only thing I knew for certain was that it didn't have a letter J in it. And that wasn't much of a clue. My brain had deleted it for no reason whatsoever.

I asked friends for advice. One told me to close my eyes and visualise my fingers on the keypad. Trouble is, I'm so scared of thieves peeking over my shoulder, I've perfected the art of making my hand look like it's entering a different pin to the one it's actually entering. When I try to picture it in my mind's eye, I can't actually see what I'm doing. I've managed to fool myself within my own head.

Someone else told me the key was to stop worrying about it and go Zen. Next time you're passing a cashpoint, relax: it'll just come to you, they said. But I couldn't relax. If you forget your pin, you have two guesses at an ATM, and two guesses in a shop. A third incorrect guess incurs a block, and isn't worth risking. Fail on your first two tries and you have to wait till the following day, when your guess tally is reset. All of which makes each attempt pretty nerve-racking - like using an unforgiving and incredibly irritating pub trivia machine.

Over the past few days I've approached cashpoints with misplaced confidence, only to suffer last-minute performance anxiety. It's like trying to go at a crowded urinal, when you're wedged between two men with penises the size of curtain rods, pissing away like horses. Just as a shy bladder refuses to wee, my brain refuses to dislodge the number. It won't come out. Not a drop. I'm impotent.

This morning I gave in and called the bank, ashamed. Sensibly, they wouldn't read my pin out over the phone, but offered to post a reminder. But because they're a bank, and banks work to an infuriating Twilight Zone calendar in which any task that would normally take five minutes in our dimension suddenly takes five to 10 "working days", I'm currently operating in that unsettling limbo familiar to anyone who's lost a wallet; you become a social outcast, carrying ID into your home branch and begging for some old-fashioned banknotes to tide you over.

Inconvenience aside, what's creeped me out is the thudding blank hole in my head where the number used to live. It can't be possible to completely forget something so familiar. Perhaps it was stolen. Perhaps someone hacked into my mind while I was dreaming and sucked it away through a pipe. Or perhaps this is stage one of my inevitable descent into thrashing, bewildered madness. What am I going to forget next? How to chew food?

In the meantime, if anyone's got any hints on lost-memory retrieval, pass them on. I've tried everything from getting drunk to lucid dreaming, and the little bastard is still hiding in the bushes, looking on and laughing. I can sense it. But I can't see it.

· This week Charlie accidentally got drunk and speculated wildly and offensively about an ongoing news event, breaking into song as he did so, at the top of his voice, in a pub. He bought a device designed to scare spiders away from his flat electronically: "No idea if it works, but then I'm not a spider." He finished reading Clockers and wants a similar recommendation: "Brilliant."