laptop won't power on fully

I have a dell inspiron 15 5558 that has been acting funny. When I hit the power button, the fans power on, the keyboard backlights turn on and I can see the power light turn on.

After 1.5 seconds, the power light and keyboard turns off and the fans power down. I assumed the power shut off so tried to push the power button again. At this point, I heard the sound of something spinning slowing down to a stop (I assume it's the hard drive).

Pushing the power button again, the same stuff happens.

I've tried performing a long push of the power button, I've tried it with the battery out and adapter plugged in, as well as battery only. I also tried to reset the unit by holding the power while the battery and adapter is disconnected.

I managed to get it to fully boot up once early this week but the problems started again a few days ago. There's no diagnostics beeps or lights.

Thoughts on what might be the issue and what the best way to get it fixed is? Dell support is saying they can diagnose it for $59 but not sure if I want to find out how much the total cost of repair will end up being..