I'm building a new rig and the CPU fan turns but I get no display output and no post.

Setup
asrock ab350m-hdv
Ryzen 1600
2x 8GB Corsair Vengeance LPX DDR4 @3000 MHz
Vx7950 from last build around 5 years ago.
Corsair hx750 also from last build

Reseating the RAM made no difference. I tried it without any RAM and it didn't even beep?
I have read that Ryzen systems can take about 5 mins to post the first time because it's 'training' the memory. I left mine for 15 mins and there was no difference.

The only thing left to try I can think of is to replace the GPU. I was going to buy another one anyway but don't have another one I can test with at the moment. It was working very recently, can anyone think of anything else I can try in the meantime?

I don't know enough about the board/BIOS but can post some suggestions as to what you can check.

Have you check all the power cable etc to make sure they are securely inserted. Although

One of the PSU output voltages might be bad. Fans typically run on 12VDC but the 5.5 or 3.3 VDC might not be present. On this board is there a 5 VDC LED for the power on circuit?

The CPU fan speed sensor might be bad and if the BIOS can not detect it running which will prevent the system from booting if so equipped/configured.

The GPU if a separate card might be bad which could prevent the system from booting.

The CPU might be bad.

The motherboard might have failed.

That board has beep codes. Even if there is no video it should still beep. On some boards you will not get any beep without memory installed. I surmise that if memory is incompatible with the board it will not beep.

I would reexamine the memory specifications for the board. If your memory needs too fast of timings or too much voltage (overclocking memory) the system might do nothing. So, try it with one stick of ram in bank 0.