Before getting in details. I have little trouble following your story because of your usage of "on". When device is on? IMHO if you press power button and something happens e.g. screen lights up, you see led then it goes on but doesn't BOOT. So your device goes on but doesn't boot it's OS. For example you could flash it but only way to flash it is to power it on :|

So first question. Did you flash it with both images: Vanilla, Fiasco?
WARNING flashing with vanilla (emmc) deletes all your personal data.Copy at least Mydocs folder from your device to computer, IIRC should be possible even if n900 is not on.