Big mistake!!!

Someone gave me a Compaq Presario F600 because they couldn't get the computer to turn on. The screen wasn't working and an external monitor had no display when connected to the laptop. I contacted Compaq and told them about the problems my laptop was having and they offered to fix it for free. Compaq paid for the shipping and repairs and I had the laptop back in 8 days.

Everything was working fine, they replaced my motherboard and thermal pad and even threw in some kind of upgrade. After receiving the laptop I started removing programs that I knew I wouldn't need and I noticed the battery was at 10% power so I went to plug in the charger... I accidently plugged in an AC/DC adapter with a 31V 1450mA LPS output by mistake.

After about 2 minutes the screen dimmed, went blank, and I realized my mistake. After unplugging the charger I tried to reboot the laptop and it appeared to have a blank screen just like it did before the repairs. I called Compaq and told them that the problem never went away. They're taking it in for repairs again, and from what I understand I wouldn't have to pay for repairs since I pretended that nothing changed.

Originally I thought I had fried the motherboard, but while writing this thread I noticed that when I turn on the laptop and shine a flashlight on the screen I can see my desktop.

I was wondering a few things, what damage did I do and will Compaq notice? I called them within 20 minutes of signing for the laptop so I’m thinking maybe they’ll just fix the screen again without digging too deep into the problem. If they do notice, how much do these repairs useally cost?