wp members crashed my website

The plugin crashing after being installed would be extremely rare. However, looking at the link you sent, it appears you are using some type of “coming soon” message – is that a plugin? It’s possible that there’s a conflict with that. Unfortunately, 500 errors are hard to diagnose because they do not give any information as to why they occurred. You might try running the site with debug mode enabled (WP_DEBUG set to true in your wp-config.php). Then if the error was caused by a PHP issue, the error would display on the page so we would have a better idea of what the problem was.