If the phone is not charging properly (i.e. shows blue NOKIA sign on white screen, then goes to a grey screen, then disappears/switches off - then the cycle repeats) then try switching the phone off and connecting in USB mode. That doesn't mean trying to get NSU detecting it in USB mode. I mean switching to flashing mode - details are here. Then try flashing it with the appropriate binary file. See details here.

I had the same issue when I first got my N900 and tried updating the firmware via NSU. Never again!