What happens when you put the correct password and username but it doesn't work?

You should try it again, but this time, type it in slowly. If that fails, someone may have hacked into your account. If you have tried it a few times, and it has failed, ask yourself how important this account was to you, depending on what information you stored in it, Ect. If its just a petty video game account, don't worry about it unless you had a lot of personal information stored on it. If its an important account, like your facebook account, then tell someone you trust and ask them what to do.