You would need to first of all ensure you are upgraded to the latest release of the software. Once you have done this, do the following
Change your site to be running PHP 8.1
Upload the following file to your server, and run it in your browser https://invisioncommunity.com/files/file/7046-invision-community-requirements-checker/
You should then be able to find out what is missing, as it is likely you are missing a requirement there.
If the above doesnt work, it may be that you have a plugin or application that isnt compatible with PHP 8. Disable those one at a time from the admin CP until your site comes back online. If you cannot access the admin CP at all, you would need to use recovery mode to disable them, which you can find information on here
Hope that helps you get the problem resolved.