Thanks for reaching out. Let's sort this out.
Quick questions: Are you pertaining to the guest mode pin? If yes, what type of Chromecast device do you have?
If the guest mode pin does not appear in the lower left corner of the TV, please open the Google Home app and make sure the guest mode is on. The pin will also appear in the Google Home app. If the issue still persists, restart the Chromecast then wait two minutes for the PIN to show up.
Note: Guest mode is not available on Chromecast with Google TV. See the info here.