Older IMac screen dead, system working, can't get external to show up

[scisport]

My mistake for not being better prepared, but here goes. Older IMac, likely around 2013. System still working great until the screen died. Tried everything, no resurrection. After booting up, I can hear enunciators working. The hard drive is intact. It has the older rectangular multi pin connector for external monitor connections. Went on line, finally found the right dongle that goes to an HDMI. The only monitor I have available is a older flat screen that accepts a DVI connector. So, I found a new connector that goes from HDMI to the DVI multi pin screw in. External is working fine. Fired up the old Mac and...nothing. Tried several keystroke commands but nothing. At this point I just want to bring it up on an external, ensure all the material has been downloaded, then scrap it. Not sure of the OS but I believe it's pre-Mojave. Any suggestions are greatly appreciated. I'll provide whatever pics or additional info needed. Thank you.