Why my mobile having line on the screen?

@hariprakash looks like a bad display. It is also possible that this is a torn ribbon cable or a bad connector. Ultimately, get a new display and while you replace it, check the connector where it plugs into. Check to make sure there are not bend, broken or corroded pins.

Since iFixit does not yet have a guide for htat, use something like [https://www.youtube.com/watch?v=1CeMA6hk... |this video for it. Replacement screens are available at plenty of online stores,. Get one that is within your geographic region. That way you do not have to worry about long shipping times or returns, in case there is something wrong with it.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.