when typing to a cell check if this number exist in a range of cells (in sheet 2)

I need help to this : When i type a number to a cell and press enter , i want to check if this number exist in a range of cells (in sheet 2) , and if exists , excel show me a message. Actually i use it for my dvd club. Number is the client code. When i writte 50 in a cell , i need from excel to check if this client own me money , and show me some message..
Thnaks