Some may experience problems with cookies and login.
Either from your desktop, right-click on the Explorer Icon and select properties or from the toolbar in the Internet Explorer, select Tools and then Internet Options
This will bring up a window as shown below, select the delete button as shown
This will then bring up another window as shown below.
Select Delete Cookies and then try logging in again. You can then delete passwords if that does not work. Also, double-check your computer's calendar and clock for accuracy because session cookies are usually "time sensitive." If you or your child, or anybody, changed the computer's calendar either on purpose or accidentally, and your computer says it's the 2100s for a year, then a whole lot of websites won't work when you try to login. Or they'll get tempermental after a while. Guess how I know!
Oooo, I was mad for a while 'cause nothing would work with trying to fix it, until I went to the Microsoft website and did a search on my problem. Be careful with using your computer's calendar to "look ahead" to future months, years, anniversaries, etc., 'cause if you accidentally okay the change that was only meant to be for the moment that you were looking ahead, then you could run into problems. It's better to use a different calendar for looking ahead, even if it's an online one, if you're prone to the occaisional embarrassing computer 'oops!'
If all of the above wasn't helpful, I would refer you to the website of whomever is the creator of the particular computer operating system that you're using, whether Microsoft, Apple, or whomever for tech help, more helpful articles than this one, etc.