iMac will not turn on if disconnected/switched off from power

For past 18 months when iMac has had to be disconnected from power it will not turn on. Initially after 30 min, next after 5 hours, next it took 2 days, next time 3 days, this time it has been 2 weeks and it is dead. No sounds at all.

Opened unit, PRAM battery fine. No power going to on switch.

Can it be the power supply?

Can you advise where I can purchase a PSU - MV1060Z126YTB power supply.

Am in Brisbane Qld Australia

Also I would like to source circuit diagrams for it as well.

Thank you

Ros