A lot of times there's a virus attached to the message. You call the number listed and they tell you they will give you the "cure" for an exorbitant amount of money. The "virus is set to activate at a specific time and makes whatever system you're using unusable until you pay up. The only way to get past it is a hard reset/reload of your operating system. Sometimes even then that doesn't work and you're basically forces to buy a new one.