Whenever you login with Sesame enabled, you are prompted to have your authorized copy of Sesame use its authentication key to login.  Underneath the entry field for your Sesame key, there is a link entitled "If you lose your Sesame device, click here to disable Sesame authentication."  Clicking this link will send an e-mail to your LastPass account e-mail address (or security e-mail address if you have set one), that will contain a link that will temporarily disable  Sesame.   If you are unable to access your e-mail for some reason, and have lost your copy of Sesame, please contact LastPass support.