If you are unable to access your iPhone because the display is not working, the first thing to try is to perform a "forced" restart.
Ref: Force restart iPhone - Apple Support
If that doesn't help, the next step would be to try to restore it. To do so, connect it to your computer. When the iPhone appears in the Finder (macOS Catalina+) or iTunes source list, click the Restore button on the Summary tab. (Note: The following will erase all of your data and media and resets all of the iPhone's settings. You will need to restore it from a previous backup from either iTunes or iCloud. Note: If you are running macOS Catalina, you will perform the same task using the Finder app instead of iTunes)
If that also does not remedy the issue, then make a Genius Bar appointment at your local Apple Store or take it to an Apple Authorized Service Provider. Alternatively, you can contact Apple Support directly. To do so, simply select the "Get Support" link in the upper right-hand side of this forum's web page.