Incorrect Username or Password
One of the most common reasons one cannot open their yahoo mail is that they forget or are typing an incorrect Username or Password. The only thing to ensure is to cross-check and then start typing them again; maybe you will be able to open your yahoo mail once you put the correct details (Username and Password).
If you know that you forget or typing the wrong username or password, you can go to forget the password or to find your username, and you need to enter one of the account recovery items listed in the Sign-in helper; follow the instructions.
Problem with the Yahoo Server
You can get to know if it is a problem with the Yahoo server by opening the other websites. If you can operate all other websites, it means the problem is with the yahoo server. Try to refresh the page or go offline and online, in case refreshing does not help.
If that worked, it’s all done. But if not, try it after some minutes or hours, as sometimes the website’s server is under maintenance due to some reasons.
Problem with the Internet Server
Maybe your Internet server is down, due to which you cannot operate any of the websites. If you cannot use other websites, it implies the problem is only with your internet connection. To sort this out, you can try clearing the cache and reloading the browser page.
If the problem persists, you can disconnect the server and try logging in again. Still, in case you are facing any problem with the server, call your service provider.
If your system is infected with a virus, it may be one reason you cannot open Yahoo mail. It is a huge possibility that a virus wrecks your system’s internal files and also your internet server, which may not target Yahoo directly but may hinder all other functionalities, including the web service and the email service.
Some other Problems
You may not be able to access your Yahoo mail because of other problems, such as you may be using an old browser, the service has recently upgraded, or in case the Wi-Fi signals are weak.