This is a situation one can easily end-up if the option ‘autologin’ is turned on. Not typing your password for a while leads to forgetting it. This is how to reset a user’s password on a Fedora 20 and Ubuntu 14.04 system using a LiveCD. There are a few other methods to do this but they have a high rate of failure. Moreover, this is a tested method so it’s pretty obvious it works.
To reset a password on these two systems, I’ve used an Ubuntu LiveCD, but you can use about any other live distribution CD as long as it’s the same arhitecture. Using a 32bit livecd on a 64bit installed system just won’t work.