Its my sister's toshiba a205-s4577 it does not boot up at all. when i plug in the ac power, the blue plug light goes on, but when i press the power button, all the lights flash for a second and then nothing. the blue plug light goes away too. i tried reseating the ram and hard drive. nothing helps so far. is it the mobo?

Hello.

The most common reasons are the battery, power adapter/cord and power jack.

If you move/wiggle the power adapter at the point where it connects to the laptop, and the power goes off and on, the power jack is the problem.

The power jack is inside the laptop and either needs soldering or replacement.

If no change, the power adapter is probably faulty and needs replacement.

Also, power off and remove the battery. If the laptop powers on with just the adapter connected, your battery is faulty.

If you are knowledgeable, remove the heatsink and apply new thermal paste.

But, it certainly could be the mobo.

Hope this helps.