You will need to do some troubleshooting to rule out the conflict of a plugin or theme with your current version of WordPress. For this you would need to access your site files either through an FTP client such as Filezilla, or through the File Manager in cPanel (your host may well have given you login details for this).
If you can access your site files through either of these routes, then you would need to navigate to /wp-content/plugins/ and rename this directory to /wp-content/plugins.hold (this will deactivate all your plugins). If this does not allow you normal access, then also switch to the default WordPress theme by changing /wp-content/themes/{your theme} to /wp-content/themes/{your theme}.hold. This should force the default Wordpress theme to be activated, providing you have one in your themes directory. Then try and access. If either/both of these methods resolves the issue then it is a case of reactivating the plugins/theme one by one until you find the conflicting one.