Access your server via FTP, SFTP, or your hosting provider's control panel, and re-name the .htaccess file in the /public_html/ directory to htbroken (removing the preceding dot is important). You may also need to set your file manager to view invisible/hidden files to see the .htaccess file.
Then, try re-saving your permalink structure at Settings/Permalinks in your admin panel. If WordPress cannot automatically edit the .htaccess file, it will provide manual instructions after saving.