If you've bought it brand new from an electronics shop, I'd highly recommend returning it and get a replacement. The problem could be a defective display and it will require a display replacement.

By just saying the screen is black, I can't tell which is the cause, because there are many different reasons: (1) If your phone has received fall damage, you'll most likely do a screen replacement. (2) If your phone has received water damage, take it to a repair center and do an ultraviolet bath (3) If you've fiddled with the brightness, try to change it, but you need a good memory. (4) Other reasons could be a defective backlight, loose connections, software errors, anything.

Check connections, detach and reattach the display flat. If that doesn't work, try a different screen.

This is a motherboard problem. You have to take it to a professional repair center. Plus it's an ipad so opening on your own will be a great pain unless you're a repair technician.

In this phone you have more than 1 microphone. The one you're mentioning is the one on the sub board (or daughterboard as some people call it). You either need to replace the microphone on the sub board or replace the whole sub board.

You need to open the phone and try to detach and reconnect the screen and see if it solves. If it doesn't it's either a problem with the screen or the motherboard's screen connector needs to be replaced or the bridge between the screen and the motherboard is damaged.

The operating system is corrupted. The best thing to do is to reinstall a fresh copy of Windows. (1) To recover data: You need to extract the SSD or HDD and use a SATA to USB connector and then go to your disk --> select Users folder --> Get your username that you usually see on the Windows login screen --> Grant permissions --> Extract whatever you need. (2) Once you've extracted whatever information, you need to format the disk in "Create and Format hard disk partitions". Once you've done that, put the SSD or HDD back in. (3) Reinstall a fresh copy of Windows: You need a USB at least 8 GB to install Windows Media Creation Tool, you can get it on the official Microsoft page. Format the USB to FAT32 (This will delete all the files within the USB) and run the .exe program of the Windows Media Creation Tool to make it bootable. (4) Now Insert the USB key into the USB port and then once it boots to the USB select the 64 bit variant and follow the instructions.

Control connection, detach and reattach or try another screen replacement, in case it's faulty

Check the RAM. The most common issue with HP computers is the RAM. Try to test with a single RAM in different SODIMM ports. If that doesn't work change the RAM.

From what you described it sounds like a big problem that can't be resolved. Since the cable and the sub board are ok the only big problem is the motherboard port. If the circuit between the motherboad and the main cable port doesn't register then there's a probability that the port is dead, you phone is bricked. Best thing you can do is to get a new motherboard. A main board to be exact. With the battery that is remaning transfer ASAP your data. But by the time your reading this I think it's scrapped for sure.