If your Mac is not booting after the attempted repairs by First Aid, then my guess is the hard drive is failing. Your screenshots show no valid partitions on your internal hard drive. You can try running the Apple Diagnostics to see if any hardware issues are detected, but unfortunately the diagnostics do not detect all drive failures. I can provide instructions for creating and using a bootable Linux USB stick to check the health of the hard drive if the diagnostics do not show a bad drive.
Otherwise you can use this Apple article for further troubleshooting which will require you to perform a clean install of macOS since your internal drive is not showing any partitions or volumes. However, erasing the drive for a clean install will destroy all data on the drive. I hope you have a backup of your data, otherwise you will need to contact a professional data recovery service to attempt to recover the data.
If your Mac doesn't start up all the way - Apple Support
Specifically you are now at step# 4 of this article:
If your Mac starts up to a question mark - Apple Support
You can try installing macOS to an external USB3 SSD and boot from the external SSD to continue using the iMac as long as the drive failure does not interfere while booted from the external SSD.
You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.
