The problem is solved.  Here is the original question:
==============================================================================
This afternoon, a strange thing happened in my 3/50.  While I was doing
normal work on the 3/50, the system suddenly died.  After I tried to
reboot the machine, it always complained:
        -: stty: not found
Did anyone happen to meet the same type of trouble before?  Any help will
be appreciated.
Thanks in advance.
Shannon Yeh
-----------
==============================================================================
Solution:
As many have pointed out, the file system is not mounted properly
causing various files including /usr/bin/stty to be unaccessible.
It turns out that the /etc/rc.boot file is corrupted and causes
mounting problems.
Many THANKS for the help from the following people:
  Wolfgang Thaler - Sun Germany Professional Services Consulting - Munic
  Mark Plotnick <mp@allegra.att.COM>
  Steve Harris <etnibsd!vsh@uunet.UU.NET>
  tsao@tcad.ee.ufl.EDU
  Urszula Frydman  <urszula@garnet.berkeley.EDU>
  Hal Stern - Consultant <stern@East.Sun.COM>
  Ed Arnold <era@niwot.scd.ucar.EDU>
  vanandel@stout.atd.ucar.EDU
  Guy Harris <auspex!guy@uunet.UU.NET>
This archive was generated by hypermail 2.1.2 : Fri Sep 28 2001 - 23:06:13 CDT