Computer won't turn on, solid orange (amber) light

I know this topic has been stated before, but there are no threads that explain the step by step workflow to discovering the cause of the problem.

This should be our goal.

The probIem:

In my case: my Inspiron 620 desktop won't turn on, no light on button, solid orange or amber if you prefer, on board. No whir, no fan. Worked fine yesterday.

It's happened before, quite often actually, in fact, part of me thinks that if I go upstairs and press the button right now, it would whir to life.

Sometimes it goes months without being weird, then it becomes unresponsive, save the steady light, that orange light, like a heartbeat, "I'm still here..."

A bit ago I had to get a new power unit, but I attributed that to a storm, at the time I was fortunate enough to still have the computer under warranty, but this is no longer the case. When I spoke with the very pleasant man on the phone, he walked me through some steps to figure out what the problem was.

I no longer remember those steps.

So the question is: What are the steps to determining the problem my computer is having?

Thank you