My newly built PC has stopped working - please help!

Setup

• Processor: Intel Core i5-6500 3.20GHz (Skylake) Socket LGA1151 Processor
• PSU: Corsair CXM750 ATX/EPS Semi-Modular 80 PLUS Bronze Power Supply Unit, 750 W
• Motherboard: GIGABYTE Z170-HD3
• RAM: 2x4GB 2133MHz Kingston HyperX Fury DDR4 Memory
• Storage: 250GB Samsung EVO 850 SSD and a 500GB HDD
• OS: Windows 10 Pro

Hi

I built my PC about a year ago and it had been working well until recently when it just stopped booting up. I asked my friend (who is IT savvy) and he suggested that I test/ replace my PSU. I was able to get a free replacement PSU from Amazon but even after connecting that (the one outside the tower in the photo), the PC doesn't seem to boot up. No lights, no sound .. nothing. The only thing that makes a little bit of noise is the new PSU. Is my motherboard gone? Or is it my power-switch that I need to fix? Or is it something else completely. Please help! I'll be very grateful.

Some photos attached for reference.

 

Try clearing the CMOS.

 

Apologies, I thought the original thread hadn't posted successfully. I didn't realise that it goes through moderation before being posted.
Thanks a lot for the troubleshooting steps. I'll give it a go today.

 

Will try, thanks.