As of today, Sept. 11, 2014, Yahoo is up and running with no problems. It has been more than a week since Yahoo has experienced any major issues.
Before determining if Yahoo is down or having problems, it is important to check the spelling and accuracy of the URL. Check your Internet connection or computer settings if the URL is correct but the site still appears to be down. If the URL is correct and there are no issues with your Internet connection or computer settings, many websites can test service activity by directly "pinging" the website. This allows you to determine if a site's issues are unique to you or if everyone is currently having access problems.