Go here on your computer

C:\Users\Alan\Documents\maya\2012-x64\prefs. Use your name for the user.

And change the name of your prefs folder to OLDprefs. Thus saving them if they are not corrupt.

Re start Maya, you should get a dialogue asking if you want to use the old prefs or make new ones.

Clikc on make new ones and try out your shader again.

We used this trick at Autodesk all the time for wierd unexplainable crashes.

For some reason Maya's pref files get corrupt from time to time.