my scene view has accidently changed to a setting where it only shows axis. I dont know what has happened. How to bring it back to view where it shows all game objects like before? I have circled the problem part in image.

It is very simple First change scene view from "Shaded" to "textured" by clicking on shaded written below Scene. Then do following

1. Select MainCamera from Hierarchy by clicking on it

2. Then click on "GameObject" menu in toolbar

3. Finally click on "Align view to selected"