Computer started flashing colors?

[Maria]

I pluged in my computer and it started showing colors, kindof like when a tv is on with no signal, but more colorful. I started pressing stuff like ctrl+alt+delete and the off button. After about 20 seconds it stoped and when I turned my computer on again it started normaly, but with the "do you want to run your computer in safe mode" option. All I know is that I pluged my computer in. My brother and sister were by the computer at the time, but they said they didn't do anything and I belive them.

My computer is a dell laptop
runs on windows 7
I got it in August.

WHAT HAPPEND AND WILL IT HAPPEN AGAIN?