Black screen
Help getting old data off when iMac has black screen
Earlier Mac models
Question marked as
What year model iMac and what macOS is it running?
see > Identify your iMac model - Apple Support
What happened when you tried the steps at > If your Mac starts up to a blank screen - Apple Support
Do you have a Time Machine or other backup on an external hard drive?
see > Other ways to back up and restore files - Apple Support