Steady light means the battery is too low for phone to boot. The hardware is performing emergency charge. Leave it for 32 minutes or so, if it's still steady yellow then unplug and remove battery for a minute or so, put it back in, plug charger in and repeat. Use the original wallcharger, don't hook it up through USB to a computer.

If it doesn't get you anywhere and you aren't an electrical engineer I'd then go to Nokia Care for help with determining whether you need a new battery or if the fault is with your n900 or with the wallcharger.