Today’s browsers have the ability to remember the login details for pages that you visit. If you happen to mistype those details, or you used different login details the last time you visited, the browser then remembers the wrong ones. When you next try to login, unless you keep an eye on the box, your browser will substitute the wrong details just as you press enter.
So, start entering your Username. If your browser substitutes something else, click in the Username box and delete the wrong characters and type in the correct ones.
Similarly, if your browser puts a series of xxxxx in the Password box, click into that box and delete all those xxxxxs, and type in your correct password.
Before pressing the Login button, tick the Remember Me box and your browser should now remember your correct details.
This is when a clearing of your browser’s cache will be useful, but this will erase your login details for all the other sites that you visit.