You perform a factory reset. The exact procedure changes with the make and model but this is usually how we reclaim use of the tablet. Apple products differ here a lot. Apply uses a LOST MODE that blocks use if the product is lost or stolen. You can't imagine how upset the "new owner" can get about that.
PLEASE COULD ANYONE TELL ME HOW TO SIGN IN MWHEN YOU LOST PASSWORD.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic