iMac powers up but only to sign on screen, but I cannot select start.

Do you hear the fan or startup Bong?

Just with the limited information I’m guessing the power supply has failed. But that’s just a guess! You really need to dig a bit deeper. Did you check the outlet and the power cord, try another outlet and cord. Did you have a lightning storm or other weather event that could have caused a power surge (lights flicker or anything else stop working). I would get a simple outlet checker to make sure the outlet is correctly wired. You might want to check your buildings ground and get a good surge suppressor to protect your equipment if you discover your at risk.