How can I fix or check motherboar with multimeter

Hi

I wanted to ge assistant. i have dell ispiron e1505. a few weeks ago i turned on and the screen was blue with some vertical lines. i went on you tube and came across videos that showed how to dissassembly. i discconected all major component ( cmos battery,disc drive, inverter wlan card...etc) this was my attemp to eliminate to find the source creating a problem.I knew that as I added componets if there was any changes that the last part i added most likely was what caused it. To my surprice when I put it back together my screen went from having lines to now staying black. I get no boot sound,no fan the laptop powers on for few second then turns off.My laptop does show led1 solid green led 2 and 3 flash green.(left side led light). i also switch the ram to the seond slot but get nothing. do you think If I use a bootable usb to jump would solve it? The dc jack on laptop and on cord are the correct voltage.by the way it was running on window vista