It's a power rail failure -- open up the system and verify that you haven't spread heatsink compound on the mainboard.  If you have, carefully clean it with alcohol and allow it to dry.  Then reassemble and see if the system will then POST.  If it still won't, you're in need of a replacement system board.  
Hi, I just replaced my thermal paste and suddenly my laptop won’t turn on and blink, 3 amber light and 5 white light. When referring to Vostro 5468 manual, the code wasn’t exist. But when referring to Dell website, it says the code is power sequence failure. What did go wrong and how to solve this issue? 
Vostro 5468 won’t turn on and LED blinking 3 amber, 5 white