Screen flashing horizontal lines after replacing RAM

Hi everyone

today I have replaced the 2 original RAM of 2GB each with other 2 of 4GB each (Kingston) for having better performance.

I have a Macbook Pro late 2011 15'', IOS 9.3.1.

I opened the computer, hoovered it very carefully, with the lowest power and then I have removed the two RAM.

I did fit the new ones, the machine was working perfectly for 20 minutes then the screen went black (the electricity was running in it, I could see some lights on the bottom), I did shut down with the button, switch it on again and horizontal lines appeared.

I switch it off/on another time and nothing changed. I replaced the new memory with the old ones and I still have the horizontal lines running through the monitor (white, grey, black).

Any idea about what is happening and how to fix it?

Thank you in advance!