Question Acer Aspire 5742G: power button lights up, the noise of it working starts with the fan for few seconds. Then it cuts off with a ticking sound..why?

[Sha@007]

Acer Aspire 5742G is my laptop . When I press the power button, it lights up, and the noise of it working starts. The fan also works for a few seconds, then it cuts off with a ticking sound inside the laptop. It continues turning on and off in the same way. The system is not booting, and the display is not working. Previously, the laptop was working fine. I just replaced a damaged speaker, so I had to open up the panel. While opening the panel, I removed the RAM and hard disk to clean them when I found dust. Then I reinstalled them properly the same way they were before. I unplugged only 4 cables while opening the panel: one from the power button to the system, another for the trackpad, and the others for the keyboard and speaker. After replacing the speaker, I properly reconnected all these connectors.

Please anyone help....!!!!