imac won't boot

Try an SMC reset:

How to reset the System Management Controller (SMC) on your Mac - Apple Support

Try starting up in Safe Mode:

Use safe mode to isolate issues with your Mac - Apple Support

If neither of those work boot into Recovery and Reinstall Mac OS.

About macOS Recovery - Apple Support

Try Single User Mode (hold command-s keys)

At the prompt, enter this command:

/sbin/fsck -fy

Press the Return key.

This will check your directory and report that it was OK or repaired. Then enter this command to startup normally:

reboot

Run Apple Diagnostics  

Press and hold the d key on startup:

https://support.apple.com/en-us/HT202731

If none of those work make a Genius Bar appointment for a free diagnostic:

Genius Bar Reservation and Apple Support Options - Apple

If it loads then run Disk Utility > First Aid. If that reports OK then choose to reinstall Mac OS. This will be an install "in place" and shouldn't affect any of your own files. But that said, you should always have a backup.

Depending on the age of your Mac either Apple or an AASP (Apple Authorized Service Provider). I can't give you a cost, you have to ask them. You can bring it in to Apple for a free diagnosis. They also have more comprehensive, proprietary tests they can run than anything available on your Mac.

Try to start up in Safe mode https://support.apple.com/en-gb/HT201262

It can take up to nearly 10 minutes to boot in Safe Mode. So be sure to give it time to try.

I/O Error <Bummers!> Sounds like you will have to replace the drive.