Do you have a means to access the server as root/admin?
If that's the case, you should be able to change the password with the method described here (I found it on this page
Create the username, password for logging into the Nagios interface.
# htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin
# ls -l /usr/local/nagios/etc/htpasswd.users
My hunch it that you should be able to overwrite the existing file with this - or you could look at it first, but I'm pretty sure it'll not contain any clear text passwords (only hashes)... If the file's not where it's supposed to be, try whereis nagios
or which nagios
The recipe continues like this:
I strongly advise you handle this suggestion with extreme care! I explicitly refuse any kind of responsibility!
Make it available for the user who runs httpd.
Restart the httpd service.
# chmod o+r /usr/local/nagios/etc/htpasswd.users
# service httpd restart
I'm no expert. I'd use this as a last resort; please be aware that you can cripple your Nagios
installation this way (if the password file gets botched instead of overwritten, Nagios
might refuse to run). It may be a good idea to talk to the Nagios