Access your server’s error log to learn what the error is.
Unless you’ve recently edited any PHP code, the error is likely caused by a plugin or your theme. If you cannot access admin, it’s difficult to deactivate the responsible module that way. Instead, via FTP or your hosting file manager, locate the problem module’s folder and rename it. When WP cannot find the module where it is expected, WP will deactivate it for you.
The error message you see is where PHP realized there’s a problem. It may or may not be the root cause of the problem. If it is, the message will include the file’s path, which will include the folder you’d want to rename. But if the error path points to a core WP file, then it’s not the root cause of the problem.
Without more specifics on the root cause, deactivate all plugins by renaming the /plugins/ folder. Also rename your theme’s folder so WP is forced to use a different theme. You should then be able to access WP admin. Restore your normal theme, then each plugin, one at a time. When you try to activate the problem module, WP will balk. This tells you where the problem lies. Take up the issue with the devs of the problem module through their dedicated support channel.
