On your browser it stores information from the various sites that you go to, especially ones that you log into. This is the only way that your computer remembers that you are logged in.
Sometimes those files that your browser (Explorer, Firefox, Chrome, etc) saved from before can get corrupted so you have to delete them on that specific device. You have "cookies" which are one of the types of files that remembers certain data. A place where those cookies are stored is called a "cache".
If you clear your cache and cookies on your browser on that specific device it should allow you to log in again.
In other words it's not a problem with the website. It appears to be up and running fine.
On the other hand those temporary files that I mentioned before on your device probably need to be cleaned out. If you need help doing that just Google it for your specific browser. The ability to clean it out is built into the browser. You literally just open it up and find the correct button and push it and that's it.
Edit: you can also open the browser and find the "help" section and search it for how to clear your cache and cookies.