I'm using an ASUS laptop and a Samsung monitor. Recently horizontal coloured lines have appeared on the top of the monitor screen. Why and how to fix it? 

Go. COMPUTER Plaza. They will identify the cause. Mainly monitor broken. 2000-3000 thb depending on the size and model. 

Does the laptop screen have the same "lines?"  If not,  agree with the other member that the monitor may be on the way out, however, I would try another HDMI cable first, before buying a new monitor.