no open up the outliner, it's under Window and then select Outliner .
In the outliner select the camera you are working with (most likely the perspective camera) and then hold down ctrl +A to bring up the attribute aditor and see the attributes for that camera.

Depending on the scale of your scene and your working units you might be able to set the far clipping plane to 1000 or if thats not enough try 10000 and so on until you can zoom out and see your model without it being clipped.