Your web browser (Internet Explorer, Edge, Firefox, Chrome or whatever) retains certain data every time you visit a website in its 'cache'. After a while the cache can get 'clogged up'. The usual symptoms are when a frequently used website stops responding normally. That most commonly occurs with video streaming websites (e.g. Youtube or Netflix) and with webmail sites (such as Yahoo or Gmail).
It's therefore a good idea to empty your browser's cache every so often (and certainly when you find a particular website isn't responding normally). The instructions for clearing a browser's cache are different for each browser but the link I've provided covers all of the main ones. (If you're still stuck though, please tell me which web browser you're using).