My acer laptop screen has ''coloured lines'' on it's screen,how did this happen?

LCD suffer from wear and tear just like anything else. LCD can simply fail. Also heavy impact or applying to much pressure to the screen and damage the capacitors and transistors that control display of color on the screen.

Lines can form when adjacent pixels are damaged and sorta of join together this way ll or multiple pixels are recieving a voltage in either a horizontal or vertical line or even in a bing blob on your screen.

If you are developing lines on your screen you will need to get it replaced or replace it yourself if you can, as screens tend to be several hundred dollars just for the part