You haven’t provided much information to go on but I’ll have a shot in the dark based on issues I’ve encountered previously. It’s possible the cert configuration is not completed correctly but you’ve updated the application/server to force HTTPS. In this scenario HTTP requests will be redirected to HTTPS but HTTPS isn’t working. If this makes sense based on the steps you took, I would recommend researching (i.e. google) options to revert the application/server configuration to allow HTTP. This will at least get you back to square one and you can try the cert config again.
Smarter people than me probably have better suggestions.
It’s also helps to post as much information as possible when seeking assistance.
If your application isn’t Magento, you’re posting to the wrong site