Venue 8 Pro not booting even into "Diagnostics"

As Microsoft itself recommends windows 10, I upgraded, bad idea. Choppy, unresponsive and everything involved, I wanted to go to windows 8 again, booted onto a backup (original) copy of w8.1, everything seemed normal until I figured out that I wiped every partition, this is normal when you are installing any windows because windows divides disk and all of its partitions.

When W8.1 rebooted it got stuck on the dell logo, no spinning wheel, no anything, tried to reinstall w8 but when I tried to boot into the USB key again it just showed a black screen. After many tries, I tried to open diagnostics but to my surprise not even Dell Diagnostics app opened! What can I do now? Please help me as this tablet is very important for my work :