If your iMac starts up to a blank screen, it could result from display issues (eg. The iMac's display may be faulty or disconnected), incorrect startup settings, corrupted software, malfunctioning hardware, and more. To fix this issue, check out the post offered by the Apple Official:
https://support.apple.com/en-us/HT210905
If your iMac displays a folder with a question mark at startup, it means that the computer cannot find a valid startup disk or operating system to boot from. Given this condition, check out the following post offered by the Apple Official:
https://support.apple.com/en-us/HT204323
It's worth mentioning that no matter what causes your iMac to fail to boot, your data will be at risk. You'd better recover the important data before fixing these problems by restoring your Mac from Time Machine backup if you have enabled Time Machine, restoring your Mac from APFS snapshots, using Target Disk Mode, or using data recovery software like iBoysoft Data Recovery for Mac (file retrieval from an unbootable Mac under macOS Recovery Mode without installing it on your iMac or creating a bootable drive), Stellar Data Recovery (unbootable Mac data recovery without using any external bootable devices), Tenorshare 4DDiG for Mac (unbootable Mac file recovery via creating a bootable drive), and more.