Dell 1558 doesn't power on

My Dell 1558 was not used frequently since I had 5  computers to work with.  When I turned on my Dell 1558  a month ago, I was asked to tune date and time. This happened each time when I turned it on. I found out that the reason was the 3 V CMOS battery to change.   When I changed it  the computer  didn't turn on ; power button light blinked and off. I was advised to change power button.  I did it but now there is no even light for a second. There is no power at all. I have a tester but it is not clear how to find  the proper point to measure what blocks the power supply.

I would be thankful to hear some ideas what happened and why?

 Thanks in advance.

Rafael