My hard drive not detected

Please read https://discussions.apple.com/docs/DOC-5931 and then re-write your question including critical information such as exactly what iMac you have, the version of Mac OS on the internal drive and what steps you have taken. Imac's have been in production for 20+ years so you can imagine how complete information may be helpful to us.

Ah yea' that is the Recover disk.

At this point, I would say the your iMac's hard disk has either completely failed or is no longer connected to SATA controller.

In Recovery select disk0, then Erase and Format the drive.