@briankdougherty the Dell OptiPlex 9020 comes in different form factors. We would need to know your exact model. Next we would need to know if you are using the build in video or if you are using a dedicated video card. If so, we need to know the amount of memory. NExt, tell us more about the blinking. Is it rapid or does it take a while, by that I mean does the black screen stay a long time or a short time.
It could help if you can let us know what OS you are using. I assume you are using Windows of some kind.
Most common issue would be a driver error. Uninstall the current video driver and then reinstall the newest version available. Check [https://www.dell.com/support/kbdoc/en-us/000124211/dell-bios-updates|on here] for the BIOS updates. Next you will have to see if your OS can tell you how much RAM is dedicated to your video if you are using the internal video. Increasing that might help.