Is it possible that they are off centered and just out of view? Try selecting the object in question then going to Modify > Center Pivot.

If not, then it may be possible that they shrunk beyond view. Try holding down the '+' key and see if they get bigger.

If that doesn't work, try deleting the PREFS folder under your C:/Documents and Settings/(USER)/My Documents/maya/(version #) and see if that resolves it.