Yeah most likely. Try reconnecting the lcd ribbon and also making making sure that the connector pins are intact. If that doesn’t fix it then probably you need a new one. I had this problem with my iPad mini 2 as well and i ended up replacing my lcd.
The power and volume button assembly is soldered to the main logic board, so unless you have micro soldering skills, this repair is out of the average person's ability to repair. There might be services that do the repair but I'm not sure.
The best way to test it is to make sure auto brightness is on, then when the brightness is all the way up, put the phone into sleep, put your finger over the sensor an wake up the phone. If it works, the screen should dim and get brighter when you remove your finger. (Do this in a bright room)
Yes, you could put one in. The laptop does have a mini PCI socket and this ifixit guide, HP Pavillion dm4t-2100 Touchpad Replacement goes deep enough into the computer to replace the card. Just follow the guide to step seven and the rest is pretty much self explanatory. Good Luck!