iMac starts with a ? folder

How to reinstall macOS - Apple Support

Recovery mode startup key combinations :

Command-R (⌘R): This preferred combo will boot from the recovery partition built-in to your Mac’s startup drive and offer reinstallation of the current version of the most recently installed macOS.

Option-Command-R (⌥⌘R): Reaches out to the Apple servers on the internet for startup info and may offer the latest macOS that is compatible with your Mac. It requires a solid wi-fi or ethernet connection. Also called "internet recovery".

Shift-Option-Command-R (⇧⌥⌘R) : Reaches out to the Apple servers on the internet for startup info and offers the macOS that came with your Mac, or the closest version still available. Also called "internet recovery".

You need to have a connected and functioning keyboard to invoke the Recovery options.

If you have a magic keyboard, connect it to the iMac using the supplied charging cable.

If not, dig out, borrow, buy an inexpensive generic USB keyboard and use that instead.