Step 1. Clear your browser's cache. (This fixes 90% of problems.)
In Chrome: Google Chrome Support, Clearing Browser Data
In Internet Explorer: Internet Explorer, How to Clear your Browser Cache.
In Firefox: Mozilla Support for Clearing Cache
In Opera: Opera Support for Clearing Cache
In Safari (Mac): Clearing Safari Mac's Cache
In Safari (Windows): Clearing Safari Windows's Cache
Step 2. Close and re-open the browser, begin enjoying your favorite anime again.