Most probably an empty battery, emergency charging should kick in.
Connect the charger and leave it for 30 minutes or so

Here is a solution i found on here ok...

What finally worked for me was to remove the (charged) battery, connect to PC, start flasher program in cold-flash mode (-c -h RX-51:2201 (or possibly 2101) -S usb -F <fiasco image> -f -R) and then plug in the battery. There's an exceedingly short moment where the flasher can detect the N900 and do its cold-flash thing.

Provided by TA-t3 on "Pr1.1 Has bricked my N900" give it a try ok.

i have bootmenu installed and it lets me flash fine - turn off phone hold u and then connect usb and phone will boot to the flash mode and will sit thaier happily for as long as usb is connected

I think I also have the battery problem that makes it go into emergency charging. But, when I connect it to the wall charger it keeps rebooting itself (it vibrates, the Nokia logo appears for a few seconds and then disappears) and it won't respond to anything anymore. I left it there for a while but it doesn't seem to be charging..
does this mean I need to find someone with another N900 or perhaps a 5800 to charge the battery for me?