[ubuntu-in] ubuntu9.04 authentication failed -to go to root.

[Sriranga(77yrsold)]

I am facing problem of authentication of password. extract of terminal is
reproduced below:

sriranga at ubuntu:~$ cd tesseract-ocr-read-only-408/
sriranga at ubuntu:~/tesseract-
ocr-read-only-408$ su
Password:
su: Authentication failure
sriranga at ubuntu:~/tesseract-ocr-read-only-408$

I used the* same password* for login purpose and authentication purpose  for
installing, updating etc. and never faced any problem so far. Only problem
started when I tried at terminal as su or sudo to go to root - error
displayed as "authentication failure" as indicated above. I have not changed
password from the date of installation of ubuntu9.04 till today. Request for
valuable guidance how to overcome authentication failure or re-install has
to be made? I am newbie to lunux and ubuntu.
Kindly treat this as SOS.
With regards,
-sriranga(77yrsold)
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