Calling mac experts for advise

Hey not sure where to post this, but one of my friends has been having some problems with her mac. I myself have no experience with Macs or anything Apple, so i thought you guys might be of help.
She is not very tech savvy so you might also have to make do with little info.
Here it goes:
Most of the time the pc turns on but the screen is black.
“The screen is just black and when it will finally turn on it says I must select a new Apple ID password, as soon as I have made it the screen turns off and becomes black again frown emoticon”

Not enough info. I think there's a way to boot macs into debug mode or something like that by pressing D during boot. If that works, try and get a dmesg, and show us that.

I'm no mac expert but it may be fixable by reinstalling OS X, If I remember correctly to boot from CD on macs you push and hold the Command + C keys during startup.

You should be able to backup her data using one of the utilities on the install disk, I think.

First step is to reset PRAM. press the power button, before the startup tone press command+option+p+r, continue pressing these keys until you hear the startup tone a second time, or until the tone become noticeably louder which means the settings have been reset. Depending on the model year of the Mac, you can also attempt to reset the SMC, with the machine off, press the power button and hold until the LED blinks. release the power button and wait for boot. Check back after these steps

Thanks will see what I can do. Will update if we get any further.