Whenever you login with your Yubikey enabled, you are prompted to have your Yubikey use its authentication key to login.  Underneath the entry field for your Yubikey, there is a link entitled "If you lose your Yubikey device, click here to disable Yubikey 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 your Yubikey.   If you are unable to access your e-mail for some reason, and have lost your Yubikey, please contact LastPass support.