Hi!
I assume you have a backup, so restore the backup and try upgrading to version 2.3 then 3.0 and then 3.1.*
If you have a custom installation It may be that some pages do not load correctly, so try increasing the ‘Maximum Query String (Bytes)’ to 8192 in your website server configuration file (web.config or .htaccess, or similar)
Or you can just follow this topic which is very useful for custom CMS migrations