A green line has started appearing on my monitor.
Whenever I start my computer it's there. But only twice it managed to disappear by itself but only after four or five hours of usage.
Is there any way to remove it?
This is definitely a hardware issue with your monitor.
Your next step should be to immediately contact the manufacturer to check the warranty status and, if possible get them to repair or replace it. There's not really much else you can do unfortunately.
I had a similar problem with an HP 2509m. They ended up replacing it with a refurbished unit- it wasn't possible for HP to economically repair it.
If you attempt a repair and something goes wrong, then you risk voiding the warranty. This is one of those times when your best bet is getting the manufacturer involved before reaching for the screwdriver yourself.