Hello,
I am sorry to hear you are having problems with this.
To work out the cause of the critical error I would recommend enabling WordPress debugging.
You will need to edit your wp-config.php file in the root of your site using your web hosts File Manager or an SFTP client like FileZilla.
1. Enable debugging using these steps
2. Refresh the white screen page, this should write the error to the log file
3. Open the error log in the wp-content folder and view the error, it should be timestamped to match the datetime of when you refreshed the page
If you provide the error here I will be able to assist further.
Best,
Paul