Lines Appearing On my macbook screen....?

There is a lines which are appearing on middle part of the screen vertically.....what am i supposed to do?

You don't say what year your Macbook so I can only give you a general idea of what it might be. Try a restart and if that does not work and OS X is up to date ... do you have any apple care left? That is probably a display problem - either the screen or video card is having issues.

Lisa

This sort of thing, is usually pointing towards a Graphics Card issue.
One way to check, it too connect your Mac to a External Display/TV and using the Mirroring Option under Displays, if its not on the External Display/TV then it is a Screen issue.
You could try and start up in Safe Mode and see if the problem persists:

Shut down your Mac and wait 10 seconds.
Press the power button.
Immediately after you hear the startup tone, hold down the Shift key.
You should press the Shift key as soon as possible after you hear the startup tone, but not before.
Release the Shift key when you see the gray Apple logo and progress indicator (spinning gear)

Click to expand...

and/or run a Hardware Test or run Apple Diagnostics, depending on the Yr Model of the Mac, and OS.

https://support.apple.com/en-au/HT201257