Skylakeone

This is normal. The single cable from the PSU is capable of carrying enough power for GPUs that require up to 2x8-pin connectors. Doing it this way saves cost, reduces complexity, and maximizes compatibility since the one cable its capable of powering GPUs that require everything from 1x6 pin like yours all the way up to 2x8 pin.

yes. not the other thing 

I'd start with clearing CMOS, if you haven't done that yet.

This is a list of common solutions from this thread:
https://linustechtips.com/main/topic/42440-readme-how-to-respond-to-a-no-post-or-no-power-up-situation/
 
I bolded the ones that I think are applicable here.
 

Did you plug the display into 1650S's hdmi port?

try dp
also you have ez debug lights. power the system on and look at them. whichever if there is a problem with boot, cpu, gpu or ram, the corresponding led will light up 

Thanks, I've given some of these a try but I'll make sure to do each one. 

Erm, depends.
 
If you plan on upgrading the system to a fully fledged gaming rig with overclocking and all that - yeah, 400W will be a limiting factor.
If you stick to fairly low-power GPUs (like the 1650 for example) and plan on upgrading say the CPU in the future to a 12/16-core, like the current-gen 3900X and 3950X (though I wouldn't personally stick either of those in the Pro4 without some direct airflow on the VRM heatsinks), 400W will be plenty.
 
Also, the System Power 9 would work just fine in a build like this, it's one of the better units in its price range, regarding component quality. I'd go for the Pure Power 11 mainly for the better fan, both in terms of noise and longevity, since the SP9 has a sleeve bearing fan which is not really the best there is. That's probably also one of the main reasons behind the 3 year warranty, whereas the Pure Power 11 gets a 5 year warranty.

The B450M Pro4 is really decent for the money. It's among the better B450 boards out there.
If you're not gaming, pretty much any GPU will work as a display adapter even for 1440p.
PCPartPicker Part List Type Item Price CPU AMD Ryzen 5 3600 3.6 GHz 6-Core Processor £164.90 @ AWD-IT Motherboard ASRock B450M PRO4 Micro ATX AM4 Motherboard £73.97 @ Amazon UK Memory Team T-FORCE DARK Z 32 GB (2 x 16 GB) DDR4-3200 CL16 Memory £99.99 @ Box Limited Storage Kingston A2000 500 GB M.2-2280 NVME Solid State Drive £54.99 @ Amazon UK Video Card MSI GeForce GTX 1650 4 GB VENTUS XS OC Video Card £134.95 @ AWD-IT Case Fractal Design Focus G Mini MicroATX Mini Tower Case £46.99 @ Amazon UK Power Supply be quiet! Pure Power 11 400 W 80+ Gold Certified ATX Power Supply £56.99 @ Amazon UK   Prices include shipping, taxes, rebates, and discounts     Total £632.78   Generated by PCPartPicker 2020-08-25 09:49 BST+0100   Or, if you need to get it under 600 quid:
PCPartPicker Part List Type Item Price CPU AMD Ryzen 5 3600 3.6 GHz 6-Core Processor £164.90 @ AWD-IT Motherboard ASRock B450M Pro4-F Micro ATX AM4 Motherboard £67.97 @ Amazon UK Memory Team T-FORCE DARK Z 32 GB (2 x 16 GB) DDR4-3000 CL16 Memory £94.99 @ Box Limited Storage Western Digital Blue 500 GB 2.5" Solid State Drive £49.96 @ Amazon UK Video Card MSI GeForce GTX 1650 4 GB VENTUS XS OC Video Card £134.95 @ AWD-IT Case Deepcool MATREXX 30 MicroATX Mini Tower Case £24.98 @ Scan.co.uk Power Supply be quiet! System Power 9 400 W 80+ Bronze Certified ATX Power Supply £44.13 @ Amazon UK   Prices include shipping, taxes, rebates, and discounts     Total £581.88   Generated by PCPartPicker 2020-08-25 09:49 BST+0100   I went with a 1650 since it's good enough for light gaming while also pulling very little power (it doesn't require external power, it just uses up to 75W which it can take from the PCIe slot) and it also has a hardware VP9 decoder as opposed to similarly priced RX 500 series cards, which comes in handy if you're watching something on YouTube or whatever but you don't want to lose any CPU performance.
If you're doing something more graphically demanding, like gaming, you could go with an RX 570 instead, which does offer a fair bit more performance over the 1650.

Wow, that's nearly identical to the build I currently have lol. Thanks for the tips. I wanted to go with the Pure Power 11, but I was worried of limiting myself with future upgrades with 400W. Do you think this will be an issue if I go one or two levels up in the next few years?