You need to specify the location of the library when you run the dr0inst script (and specify the full path name) e.g.

SQL>@dr0inst /u00/app/oracle/product/8.1.7/ctx/lib/libctxx8.so

If you did not, then you will need to re-create the library -
use the CREATE LIBRARY command

Also make sure that the path to the ctx library is included both in the PATH AND LD_LIBRARY_PATH variable. And if you are on HP change then extension from ".so" to ".sl" and add SHLIB_PATH to the list of env variables to be defined.

-amar