First PC build no video output?

this is my first time building a system and i just finished putting everything together, but whenever i power it up all the fans and lights all work but there is no video output and nothing will show up on my monitor. Any ideas? thanks.
also i'm still using the same hard drive that was in my old computer, and so the same drivers as before idk if that changes anything.

Double check the 4/8 pin cpu power to make sure you have it plugged in.

Does the fan spin on the GPU? I'm sure you already double checked to see that the 6pin power connections plugged in. Does the mobo have another pcie x16 slot you can try? Even if its the 4x slot, might ID a bad primary if it does work.