Step 1: Remain Calm
Don't panic, but do try to stay calm. This is not a drill. The employee is probably just on a coffee break, but in a warehouse or something.
Step 2: Call HR
Notify your HR department immediately. They'll know exactly what to do, which is probably just send a strongly worded email to the employee in question.
Step 3: Activate Emergency Protocol
Press the panic button. This will trigger an automated system that will send out an all-points bulletin to all employees, complete with a photo of the missing person and a detailed description of their outfit.