When the battery is really dead you will see nothing, also in addition the usb plug could be half broken and you will see nothing. I think in both cases the led will be on.

When the battery and N900 is not totally dead, and you see some dmesg output, but the flasher doesn't recognized it (suitable device is not found), then it could be that the boot loader is corrupted. This could be fixed by cold flashing the device.

Read this thread maybe it is interesting for you. However go for a good battery and then we can see if it is really a paperweight.