Help! Big Permalinks issue!

As a quick background, I’ve constructed and maintain a website running WordPress. I’ve been using WordPress since March of last year. Today, I tried changing the Permalinks to a custom choice. And, now when I go to the website I simply get a blank screen. When I go into Admin and try to change the Permalinks back to default, I get a blank screen as well. I’ve googled the problem and tried editing the .htaccess file, but I am having no luck. Any suggestions would be greatly appreciated!