Your receptionist should have a concealed panic button.  The button should connect to your office and set off a buzzer or flashing light or *something* that will immediately attract your attention.  Then you can decide what to do.

Where should the panic button be located?  1 option is to give it to the receptionist to put on her belt like a cell phone.  Alternatively, place it under the desk or next to the keyboard are locations I like.  I don’t like putting the panic button on the floor because a) it’s either too easy to set it off by accident or not easy to set off if you need to; b) all the movies have the button on the floor so everyone knows to look for it there; and c) sometimes the receptionist walks away from the desk.  These panic buttons aren’t very expensive, you just have to test the batteries once in a while.