I don't know how to connect an external modem to my PowerBook G4 12

SOURCE: How to boot PowerBook G4 with damaged display

Download and install "Screen Spanning Doctor" for mac (freeware, great prog). Use clamshell mode (see readme). Hook up external monitor (DVI out?). Boot normally... Keep shell closed. Works for me :>

SOURCE: Open case of Apple Powerbook G3

Several screws on bottom and the whole top case snaps off. It will take a little time on this model to gently pry. Also, I think the keyboard comes off first before removing the top cover by two finger pegs. It's been a long time since I worked on this model you you'll just have to get in and look around. Very common on this design that the power plug just needs re-soldered to the motherboard.

SOURCE: powerbook g4 going to white screen only!!

Do you have any external harddrive attached to it? If so, remove it before restarting. Try to start it up in safe mode by holding the Shift button down at startup: http://support.apple.com/kb/HT1455

SOURCE: My powerbook G4 laptop screen will not come on it

You can open the computer up and see if the ribbon cable is connected firmly. Most likely the back light went out.

SOURCE: Powerbook won't start up

If you have the original install CD or the OSX CD, boot to CD. To do that you must insert CD, restart PowerBook and hold letter "C" on keyboard while computer is starting, then take the first steps towards installing the OS, when you get to the bar on the top go to Tool>Utilities>Disk Utilities then select the drive a do a disk repair. That may fix the problem and bring your system back to booting normally.