Most of the time, error 403 is caused by a corrupted .htaccess file.
The .htaccess is a distributed configuration file, and is how Apache handles configuration changes on a per-directory basis.
WordPress uses this file to manipulate how Apache serves files from its root directory and subdirectories thereof. Most notably, WP modifies this file to be able to handle pretty permalinks.
This page may be used to restore a corrupted .htaccess file (e.g. a misbehaving plugin).
Basic WP
locate the .htaccess file in your file manager and edit and replace the codes with the lines below:
# BEGIN WordPress
RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
# END WordPress
