How can I get rid of that line? Do I need to send it to service center? Can I use some canon software to manually remove those lines from pictures? This is my first DSLR and it's out of warranty! Please tell me the approximate cost of repair if required..
It appears your sensor has a 'dead line' across it. An entire line of pixels is not being read out. The sensor is not repairable and would need to be replaced.
Unfortunately, the cost of replacing a sensor in the 1100D is probably more than the camera is worth. This is because a sensor replacement is very labor intensive. The camera must be almost completely disassembled to replace the sensor.
Although the cost will vary based on where in the world you are located, a document posted by Canon UK a couple of years ago quotes a sensor replacement for the 7D (which used an APS-C sized sensor like the 1100D's) at £265 (about $355 USD). A new 1300D, the newer model that replaced the 1200D that replaced the 1100D, currently sells for around $365 USD in the U.S. A used 1100D body currently sells for around $200 in the U.S.
I'd be very surprised if you could get the sensor in your 1100D replaced by a Canon certified shop for very much less than the price of a new 1300D anywhere in the world.
If you could remove and replace the sensor unit yourself (NOT recommended if you are asking such a basic question here) the part would probably cost about 1/2 of the price of a new 1300D. But you would also need some very specialized equipment to properly align the sensor and the lens mounting flange. They need to be squared with one another so that the plane occupied by the sensor is within about 50 microns or less difference in distance between the right side and left side (and top and bottom) of the lens mounting flange.