White page or error 500?
There is a chance of 99% that its caused by a PHP error in a (new) theme or plugin. Although we do not exclude the chance of an error caused by the .htaccess and wp-config.php file.
In most of the cases this problem can be solved by visiting the hosting panel, logging in and going to the statistics. Choose for error log, latest 100 and see if you see a notice in it.
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