Q: cant log in to IMAC

Please...help us help you! iMac Model? Mac OSX version? Please look at this exceprt from Apple's article, "How toWrite a Good Question":

How to write a good question

To help other members answer your question, give as many details as you can.

• Include your product name and specs such as processor speed, memory, and storage capacity. Please do not include your Serial Number, IMEI, MEID, or other personal information.
• Provide the version numbers of your operating system and relevant applications, for example "iOS 6.0.3" or "iPhoto 9.1.2".
• Describe the problem, and include any details about what seems to cause it.
• List any troubleshooting steps you've already tried, or temporary fixes you've discovered.

You posted to the iMac PowerPC forum.  Not too many of these are in use anymore.   However if you are using one, you can tell the difference on this tip:

Is my iMac an iMac PPC or Intel?

If you are indeed using an Intel, please repost in the appropriate forum.

Newer iMacs that were bought new after October 2011 offer a self-boot options to add administrative users.

Older Macs depending on make which were released before July 20, 2011 may require their original installer disc to add an administrative user if the old one is corrupt.   Optionally you may be able to use the boot volume or disc to repair permissions to fix the administrative user.  Knowing the age of your Mac and its model name is essential to know how to find other users on your system to be able to log on or create other users.