Acer Aspire One Happy won't bbot

My Aspire One Happy had a faulty screen, we changed it with a used screen from a similar laptop. Now it won't start, when pressed the power button, the light comes on, fan starts, nothing is displayed on screen, and after few seconds, the laptop shuts down. I took the battery out, discounted from the mains, pressed the power button for 20 secons, reconnected the power, it still has the same problem. Changed the RAM, no luck. Can't update the BIOS because it doesn't stay on long enough to read the flash drive.