Wow, PIX! that makes me feeling very old : D, last time I touched a PIX was over 20 years ago. Do you want to know the IP only or would you want to reset the password as well? if you want to know the IP only and you don't want to go through the password reset procedure as shown in the link shared by M02@rt37, then one thing you could try to do is to connect the PIX inside interface to a switch and run some packet capture on that switch port. That should reveal the IP address of the PIX interface.
However, if you need to reset the password of this device then you would need to go through the steps in link provided by M02@rt37 or this link below which are pretty much the same:
Cisco PIX (500 Series) Password Recovery | PeteNetLive
The only caveat with this is that you need to know which software the PIX is running and download the same version from somewhere before you upload it to the firewall. I'm not sure if Cisco software portal still has PIX images available for download, I don't think so.
The command you are trying to issue "password newpassword" is not to reset the password from monitor mode. That command if I remember correctly serves changing the password from withing the firewall configuration mode. So, in your case that command does not help.
Out of interest, why would you want to do with this PIX? learning and studying? if so, I would highly recommend discarding that device and go with any simulator or emulator and learn from the ASAv which will defo be more suitable and matches the study materials we have nowadays. PIXs have totally different syntax and packet flow.
