Freezes and crashes during rendering may occur if your Project is 'too complex' and your hardware isn't powerful enough and/or lacks available System Memory and Virtual Memory to handle it properly.
'Too complex' refers to the following variables:
A: The number of different models you have added to the Project.
B: The 3D Point count in the top right corner of Build Mode.
C: The number and resolution of your textures/materials.
D: The Movie/Photo Effects you're using, in particular the Skylight and Hyperlight effects.
E: How many Reflection Planes you've added to the Reflection Effect.
F: How many Spotlights are set to Shadows: Dynamic in the Scene.
G: How many Spotlights you have selected in the Global Illumination Effect.
H: How many large surfaces you have assigned 3D Grass Materials to.