I have a Dell Inspirion 6400, and it's been fully reset (HDD completely wiped) and it's missing it's OS and the other...

after turning off the laptop, if you remove the bottom cover there should either be a CMOS battery slot (like the first picture) or some puck-sized shapes with a cable (like the second picture). After your find it, you can turn the laptop back on with the battery out and the BIOS should be back to factory settings and without a password. From here you can turn the laptop off and put the battery back where you found it, plugged in, close up the laptop and test it.

I believe this is happening on the laptop so try this: First, get a different display like a monitor or even your TV, and hook it up with a cable and press Win+P a few times until it shows on the other screen. if this looks good on the extra screen (no black lines or artifacting) look into inspecting the connector for your display inside the laptop or buy a new screen panel. If the extra screen still shows black lines, your GPU is dying inside your laptop and is probably time to get a new laptop. Hope this helps!

Are you aware if it works or not? If you have a external monitor or even a TV, hook it up with HDMI and press Win+P a few times until you get a signal (if it has windows installed) but I believe the BIOS might swap automatically also. if you get a display out and make sure the laptop works, check the connection inside or look into a new display. (If this doesn't work I don't have a definite fix, so don't believe anything if it fails, I only have some ideas, gl!)