Only guest user showing .main account not showing

Sounds like there may be some sort of directory or file corruption.

Restart your iMac in Recovery mode and run the Disk Utility First Aid on the startup drive.

How to repair a Mac storage device with Disk Utility - Apple Support

What macOS version is running on your iMac?

Follow up:

And I hope I'm not too tardy with this suggestion,

Before you boot into recovery to reinstall the OS, try booting your Mac in safe mode and see if you get the proper options for user login.

Safe mode will often correct weird software behavior by forcing the OS to clear caches, do disk repair and other self-maintenance and housekeeping and temporarily disables third-party mods.

How to use safe mode on your Mac - Apple Support 

You can exit safe mode by restarting your Mac normally, then reevaluate the problem.

Your big problem is you only have about 2.43GB of Free storage space as of the picture of Disk Utility First Aid. With the macOS Extended file system bad things happen when it runs out of Free storage space.....it can corrupt the files on the drive and other problems. It is also interesting that the First Aid portion you pictured, shows it resizing the Core Storage Logic Volume......that is bad and indicates a bigger problem with this volume/drive.

Try running First Aid on the Fusion Drive item.

I wonder if the Hard Drive portion of the Fusion Drive is failing.

See if you can create a new macOS admin user account using these instructions:

If you don't have another macOS admin user account, then you will need to trick macOS into running Setup Assistant again. This will involve using the command line. These particular instructions will only work on macOS up until about macOS 10.14 Mojave....different instructions are needed for macOS 10.15 to 13.x.....it is impossible to do this with macOS 14.x+.

You can do this by booting into Single User Mode by holding the Command + S keys just after powering on the computer & hearing the startup chime (hold it immediately after the chime...it may not work holding it sooner). Then at the command prompt enter the following commands making sure to press the "Return" key at the end of each line to execute each command:

fsck  -fy
mount  -uw  /
rm  -i  /var/db/.AppleSetupDone
reboot

If all these commands complete without any errors when you reboot the computer Setup Assistant should run where you can create a new admin user account. Report back here afterwards before doing anything else.

*****

If you can at least boot into macOS with an admin user account, then we have some more options available to attempt to understand what is happening here.

Do you have a good backup of this system? If not, then you that is a high priority. The easiest method may be to install a full version of macOS onto an external USB3 SSD & booting from that external drive. This would make it easier to backup of the data if needed and to try to troubleshoot a few things as well. It also gives you the option to perform a clean install. If the internal Hard Drive portion of the Fusion Drive is failing, then you will need to boot from an external USB3 SSD if you want to continue using this iMac anyway.

That 2017 iMac can run any version of macOS between 10.12 Sierra and macOS 13 Ventura.

Which OS is it running?

You were able to reboot the Mac in recovery mode to run the Disk Utility First Aid, yes?

If so, do so again and select "Reinstall macOS..." there on the main recovery screen. This process will simply lay a new copy of the existing OS on the startup drive, replacing any damaged or missing OS files. It should not affect your user files.

gowtham251 wrote:

[...]

i tried reinstall mac os in recovery mode,it shows error "recovery server cannot be contacted "

This is a common occurrence with High Sierra downloads for some time, now, and Apple don't appear willing to put any effort into resolving the situation. You might find help for it here: https://mrmacintosh.com/how-to-fix-the-recovery-server-could-not-be-contacted-error-high-sierra-recovery-is-still-online-but-broken/

Look specifically at Fix #3.

Otherwise there are options for installing a macOS version newer than High Sierra.

Do you have backups of your data?

You can set the date and time using the Terminal app when booted in recovery mode.

For example, this command:

sudo date 1214081025

– sets the time and date to Dec 14th, 08:10 a.m., 2025.

Adjust your input to match your current date and time.

You said previously that Disk Utility First Aid routine was not working.

Please tell us what happens when you run First Aid. Please post a pic of the screen results if you can.