you must select the type of extrusion!!
I found out there was a line which was not connected, so that was why it did not generate a solid. I have completed the part now. Thanks
As per the image, all lines are connected. what you did ; you select surface extrusion. You will find just above placement option two blocks are there. One of this is for solid modeling and another for surface modelling and in you image you were selected surface modeling. That's what the problem.
click solid modeling in extrusion intsead of surface...it will convert to solid after that.