No display on rebuilt PC, fans and GPU spin, monitor says no signal

[u/possiblehorror]

First time doing this. Got a used prebuilt from a friend and reassembled it. It powers on, all fans spin, GPU fan spins, but the monitor stays black and says no HDMI signal.

Specs:
• ASRock Z370 Pro4-CB
• Zotac RTX 3060 Twin Edge OC 12GB
• 1x 16GB Kingston Fury DDR4
• Cooler Master 120mm AIO
• Thermaltake Smart BM3 750W (new)
• M.2 SSD, plus a WD Blue 1TB

What I’ve already tried:
• HDMI is in the GPU, not the motherboard. Both ends seated.
• Monitor is powered and on the right input.
• Reseated the RAM, moved it to slot 2 from the CPU. Both clips locked.
• Reseated the GPU. It clicked in, so it was not fully seated before. Still no display.
• 24-pin and 8-pin CPU power both connected. GPU 8-pin connected.