When you power up the PC do you see the usual on-screen messages (before Windows loads) or does the screen simply stay blank?
I'm beginning to suspect though that there might be a hardware fault, so you might end up having to return the PC.
If you're seeking a cheap refurbished PC I thoroughly recommend this company:
https://www.accomputerwarehouse.com/copy-of-new
(The HP ProDesk 6305 is cracking good value, at £119, for a computer with solid state drive).
If you're not seeing any on-screen messages at all then the fault doesn't lie with Windows. (i.e. the PC isn't going into sleep mode, as that would happen after the boot process).