Laptop won't turn on after RAM upgrade

Hello!

I have Acer Aspire One D257 with Windows 7 installed. Everything worked absolutely fine. I wanted to upgrade the RAM so I changed the 1GB DDR3 Stick to 4GB DDR3 Stick. Now, after I click the power switch I can hear the fan spinning but there is no display on internal or external monitor and I hear no sound. Only sound is a strong beep that happens when I unplug or plug in the power cord.

What have I done wrong? What should I do? Did I do something to the motherboard? When si was changing RAM I had to complettly disassemble the laptop, including motherboard.

Thank you very much for help.