First, have you contacted the authors of the theme to get support from them once you saw the server issue after activating the theme? What did they say?
If they don’t provide support for their theme, let’s start tracing the root cause: what was the server issue? What’s in the logs exactly? What’s the theme name?
Don’t follow blindly instructions on google, and if you do, describe exactly what you’ve done so we get a clue (or link to the instructions you followed.)
I hope you have a backup of your site that you can restore and start fresh (or that you can trace back your steps to bring your WordPress site to a working state.) Once you have the site back working, you can start again fixing this new theme.