Depends on the VI. All the shipping examples (Help>Find Examples) have a block diagram (the code part) that can be viewed. Some of the functions on the functions palette are VIs with block diagrams and some a basic functions that you cannot view the code. Programs written by someone else may have the block diagram password protected or removed. Your question is a little like 'how high is up?'.
Based on your question, you may not know what a block diagram or function palette is. You really should start at http://www.ni.com/academic/lv_training/how_learn_lv.htm and at the very least, take the 3 hour introductory class so you can ask the correct question and understand the answer.
