I would check your hardware configuration.
Can you list your video card, ram, version of DirectX and what your settings for JK are in the Video settings?
Updating to 1.01 might solve your problem if you are using a Voodoo2 based card, but if not, post back and perhaps we can solve the mystery.
Note that the "whiting out" of the screen in Level 21 denotes that you have lost, as Jerec has taken all the power of the valley and killed you, so the game is supposed to end, but it shouldn't kick you out of JK to Windows (this has never happened in my experience.. it should let you try again, as with any time you die, or reload the level).
There is another MotS walkthrough page that was recently brought to my attention, that goes beyond Trail's Guide (Trail's is still an excellent resource btw) where MotS is concerned and may be of interest: