If N900 shows static (not blinking) amber/orange LED, the device is on emergency charge mode. If you have a decent setup where you know there's not a likely chance the device will go into reboot loops (i.e. showing nokia logo nonstop) then in theory you should let the device charge for at least a good 20 minutes. The case mainly applies if you are running stock setup (with no fancy bootloaders such as u-boot, multiboot, etc and/or having installed things like power-kernel etc). Charging for a good 20 minutes then try turning the device on.

If say you let the device charge for about 5-10 minutes in emergency charging mode the device boots up but you kept getting nokia logo, then you have a boot loop. Boot loops with minimally charged battery are generally not easily fixable (some could just be coincidential bug that may allow the device to boot up normally). You have basically three choices:
1) Use another battery, if you can find a donor device/phone that could charge N900's battery use that to charge N900's battery (ideally fully). Otherwise get a battery that has at least 50% charge.
2) Hack up a way so that your N900 will boot without a battery, there are a few ways but they all require skills in electronics. Unless you're lucky to know of someone that has those prototype "flashing jig" and/or access to USB TTY converter.
3) Keep having N900 connected to wall charger until the bug manages to give in (if ever).

In cases 1 and 2, if you keep seeing N900 going into constant bootloops for about 3-4 times your best plan is to quickly get N900 into USB mode so that you can flash. Ideally have your computer setup and ready to flash especially if your N900 is still running on batteries.

The other possibility would be that the battery itself is either dying or dead, they will continuously hold less charge over time. The only way to tell is to get another battery that is either new or has around 50% charge.

I see there's still people using multiboot knowing the risks that multiboot does with constantly wears the level of the flash device. It probably won't be long before your device has issues with flash memory before that module dies. The current author of kernel-power does not recommend using multiboot and already has tried to make an attempt in making people avoiding multiboots. If you're not going to heed that message, you can learn the consequence through the hard way.

I'd also like to add as a note that no computer will be able to detect nor properly flash N900 if the device was in emergency charging mode. Emergency charging mode only holds ~5% of battery charge which should be adequate for NOLO/X-Loader as well as the kernel to boot up properly. Once the kernel boots up properly, it will load up a kernel module which enables proper battery charging from whatever the remaining battery charge was held (obviously it will fail if battery charge shows 1%). A ~5% charge is nowhere near suitable charge level for flashing and/or expecting a good flash result.