HP Pavilion All-In-One not powering up.

[Idris4]

I powered down and disconnected the system for about a week to redecorate the room.

When I plugged it back, it wouldn't power up.

The wall socket is live - tested with a desktop lamp.

Check and swapped the fuse in the plug - fine.

Noticed that the power adapter feels cool to touch even when plugged in for over 3 days.

I haven't got a spare power adapter for this unit.

[WAWood]

@Idris4 

Unless you have access to a Multimeter, you won't be able to test the power adapter -- and without that working, you're not going to be able to run diagnostics to hunt down hardware problems.

You will probably have to take this into a PC repair place to see if they can test the adapter and provide you another one to get your PC running.