About 3 months ago I upgraded my PC, keeping only the case which housed my new parts. I kept the parts knowing I'd use them when I moved out as I'd leave the new build to my younger brother, which brings us to today. I've put together the old build but I can't get it to boot. The ASUS motherboard shows the orange light for BOOT_DEVICE_LED and code 99. I've tried resetting the CMOS, unplugging all connections, trying with and without GPU but I'm not making progress. Often nothing will show up on the monitor (I've got HDMI plugged into the mobo currently, tried plugging into GPU previously), but sometimes it will show the screen attached. I've never had a RAID setup. The attached hard drives don't have an OS, I was going to do this if it booted up lol. The mouse and keyboard are wired. I'll list the parts below for reference as well. Again, these parts all worked perfectly in March. I don't have access to my brother's computer/parts as we live in different cities. I've googled and tried various suggestions/solutions on the Asus forums, these forums etc which hasn't gotten me anywhere.
Pressing F1 or any key doesn't do anything. The LEDs on my mouse light up but not on the keyboard (I've tried other USB ports but still nothing).
CPU: Intel Core i5-6600K 3.5 GHz Quad-Core Processor
Cooler/fan: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler
Mobo: Asus MAXIMUS VIII HERO ATX LGA1151 Motherboard
RAM: Crucial Ballistix Sport 8 GB (2 x 4 GB) DDR4-2400 CL16 Memory
PSU: Corsair TX 650 W 80+ Bronze Certified ATX Power Supply
GPU: Asus 1070
SSD: Samsung 840 Evo 120 GB 2.5" Solid State Drive
HDD: Western Digital Caviar Black 640 GB 3.5" 7200RPM Internal Hard Drive
Case: Cooler Master CM 690 ATX Mid Tower Case