undefined reference to Struct

Code:

breachmonitor.cpp:(.text+0x17ce): undefined reference to `BreachMonitor::Breach::Breach()'

Can anyone please tell me what does it mean? I have created a struct and then declared its variable like

Code:

Breach wb;

and it isnt compiling. The structure has default constructor (no arguments) and another one for initialization. It has also got two methods.

Code:

bool operator==(const Breach& p) const
bool operator<(const Breach& p) const