I put the SSD card into an external case and connected it to the iMac cloned the internal drive to it and started up with the SSD and for ever have used the external as the startup drive the internal drive is on it’s last legs so I don’t even use it.
Hold the option key down and see if it sees the boot drive. You could be having hard drive failure. Also try to insert a Startup CD/DVD/USB drive with a system on it. let us know if it does anything with that. The drive may need the permissions reset.