I got an N900 two days ago (after major issues with the N95 and N96 I was weary of getting one); it's been working great until just: my battery had died after downloading and installing the large Debain image and now, when I try to power on the device, with or without the charging lead in, the Nokia logo shows, I see the dots on the screen but the screen doesn't come on; it stays a dim black but still responds to touch.

Also, when I try to charge my phone (with the USB in my computer or mains plug socket) a USB icon appears in the top right; is this normal, as I've read this means it is in flashing mode.

What should I try?
I've read that I should try flashing the EEMC(?)

Would someone please be kind enough to spoon feed me through the process as I a quite new to this, thank you.

http://wiki.maemo.org/Updating_the_tablet_firmware

If it starts to boot it is not dead. just something it does not like.
Try flashing the firmware again before fllashing emmc.

It does sound like a good candidate for reflashing. The documentation you were referred to says that N900 users don't have to flash the eMMC, but that is false, by the way. N900 users flash the eMMC left and right.