Unity editor not working properly!

My unity scene tab is not working properly! it working fine before but dont know what happened to it. Screen shoot is attached below!!It is not displaying properly. Unity version is 4.3.4[31922-screenshot+(140).png|31922]

Looks like you’ve just moved too far away from your scene. Hit ‘F’, which is the Frame Selected command. This will zoom the camera in to your scene again.