Assuming you know the amp is working and properly adjusted, and your guitar lead has no duff connections then the guitar needs checking. first port of call is to undo the two little screw that hold the cable connector in place, GENTLY pull the metalwork away from the guitar body and take a peek at the connections underneath, give them a little tug, even better, use a continuity tester or the Ohms range of a multimeter to check there are no open or short circuits between the various parts. You should be able to leave the guitar lead plugged into the socket whilst you remove the cover and visually and manually the solder connections on the underside of the cover. Test for electrical continuity from the tip of the feed cable at the amplifier end to the corresponding terminal at the connector, then repeat the check with the ground connection. There are only two wires, signal, and ground.
If the fault is not here you will have to remove the front cover and check all the connections underneath.
Arthur
February 2008