|Our problem here is that we hear every possible reason you can think of why someone can't access a computer. We can't know who is honest.|
If there was a password on your account how did someone else gain access to your account to change it?
If the computer is yours then you should know the administrator password. Boot into Windows safe mode using F8 at start up. You may be prompted for a administrator password. If you never set one then try hitting enter. Once in Windows safe mode you should be able to fix things.