You can reflash the kernel (as per wiki reflashing instructions) as normal.

Just ignore uboot, or plug the N900 in and don't bother turning it on.

I had this happen once (I made my own uboot+kernel image, used a zImage - had to be converted to uImage, and it wouldn't be able to load the zImage, so it would get stuck on the U Boot screen). I don't remember literally every second of what I did, but I do remember that I reflashed the stock PR1.3 kernel (just kernel, no need to wipe everything else), using the flasher utility, as normal. I don't remember if I let it boot into uboot or not. I'm pretty sure if it tried to start uboot, I just shut it down (pulling battery was the only way), and put it back in, and DIDN'T turn it on.

If everything is normal, your N900 should still be ABLE to enter flashing mode the moment the flasher utility tries to flash it. The hold-u-key process is just an extra layer of just-in-case precaution. It's not necessary.

In the future, look up what your N900's hardware revision number is, and check which uboot version you're using (the latest one including Matan's, e-yes', and other patches doesn't work on one of the hardware revisions).

Also, remember if you're making your own kernel+uboot images, that the extracted zImage has to be made into a uImage using "mkimage" first.

- Edit -

If your battery is horribly low, however, I don't know how to help you. You'd either have to charge it in another Nokia phone that takes the same battery, or in a special charger. Or you might get lucky and the N900 can get enough power from the USB port during flashing even with no energy what-so-ever.

In my experience, when the kernel image attached to u-boot is good, the N900 will start charging automatically. So you'll plug in the charger, uboot will come on, then screen will go dark (but still show the uboot screen, just without backlight), but the N900 will charge (this is just like when the N900 is off, and you plug it in, and it starts charging. What it does, is loads the kernel partway, then drops you into a basic charging mode - when uboot intercepts the kernel first, it takes over the screen long enough to display itself, but then when it launches the kernel, it sees that it got booted from a charger-plugged-in event, and just drops into charging mode). From here, you can let the battery charge long enough to boot normally.

However, if the kernel ISN'T loading, I'm not sure how much it can charge without it. I THINK the hardware has a very basic charging capability beneath what is normally controlled by BME. So it MAY be able to charge, even if it doesn't show anything, enough to boot/flash - but no guarantees on that.