Hi,

I'm going to give this one last shot before I throw the whole thing in the trash .  I have a Toshiba C55-A5105 that was working fine until a month ago or so.  It all of a sudden would START and would shut down at various times. Sometimes it would boot successfully into Windows 10 for a few minutes, sometimes it would occur during the startup, but each time I'd get a different error message and a reboot.  Eventually it wouldn't boot at all and I'd just get a black SCREEN.

I know the battery wouldn't hold a charge, so my first assumption was some kind of power surge and a motherboard issue.  But I tried various things first to no avail:

1.  Tried 3 different hard drives, none successfully
2.  Tried 3 different RAM cards
3.  REPLACED battery and tried different ac adapter

The power light comes on no problem, the DVD drive spins right up, but I only get a black screen (no boot at all), and the HDD light doesn't come on.

So, I decided to replace the motherboard.  Bought a new motherboard and processor for the C55-A5105.  I carefully replaced the old one and successfully put it back together.  Again tried various COMBINATIONS of RAM and a few different hard drives.  Still a blank screen and no HDD light.  I tried using an external monitor to see if it was a display issue and no luck.  I've tried booting with AC Adapter and with a charged battery as well.  Even tried the Shift-F8-Power combination.  I've let the laptop sit for hours with the power on, but never does it show a boot screen or any display whatsoever.  I even tried pulling out the DVD drive and booted without it, but no change.  What else could I possibly try or replace to get this thing running again?

Thanks for the help!

-RobHi Rob,
I just wanted to let you know if you haven't trashed the computer yet, that I received a C55-A5105 that would not power on (I repair laptops).  It had the same problems. I replaced the motherboard and checked all of the power connections.  The answer to the problem in this case was a defective USB board.  This laptop has 3 usb connections (it was the one on the left side with only one usb port).  That port was interfering enough with the power that the computer would not boot.  Once I removed it, the computer booted normally.
 
HOPE this helps
Carla