There could be many reasons why the fence is not working. If you have a wireless dog fence, physical obstacles are a common reason why the signal is not picked up by the collar. As for the in-ground systems, you should first check if there are any physical damages to the wire or the transmitter. If there are not, the battery in the collar might need a replacement. If that doesn't work, then you maybe have too much wire for the transmitter, and the signal is not effectively broadcasted. You also want to check if other electric signals in the environment are interfering with your electromagnetic field.