My computer was running completely fine yesterday, and this morning I went to boot it up and- black screen. I heard the 'dong' sound but nothing happened. I plugged an external monitor and nothing. So, I've tried booting it up now multiple times- sometimes the 'dong' sounds and sometimes it doesn't, the sleep light comes on dim and you can hear the hard drive spinning, but nothing on the screen. I tried to reset the pram (hold down Apple-Alt-P-R after startup. right?) but haven't gotten any registering beeps. About a month and a half ago I was having major graphical problems (every other pixel was a white vertical line) but this suddenly 'fixed itself' one day and has been great for a few weeks, no issues. The only thing I did differently yesterday was to boot into my Boot Camp XP partition, which I haven't done since the problems started. Could this be a catalyst for the issue? Could it be the Nvidia chip or logic board or (PLEASE!) something else? And what else can I try to fix/figure out the problem? I'd like to exhaust all options before taking it into the Apple store, as my Applecare has run out. Thanks so much for any help!