I think the problem might be the switch which momentarily "shorts" pin 16 of the 24pin Motherboard connector to ground in order to start up the power supply . See the web page:- http://pinouts.ru/Power/atxv2pinout.shtml. If you feel able, carefully try shorting the leads to the switch momentarily (Mains supply plugged in).
What is Amazon Go and will it come to the UK? The store without checkouts or queues