Diagnostic Tree: Short
Finding a Short Circuit in Your Home
If you have reached this page without beginning at the Start of the diagnostic tree, you may do better to start there.
A hot-to-ground short (breaker-tripping ground-fault) is more common than a hot-to-neutral short; it can be helpful to know which you have, and there is a way to know. But let's consider how to attack the short in any case. Most breakers can stand up to repeated shorting, so be ready to keep resetting the one in question. (Don't HOLD the breaker on, however; just firmly push it more off, then firmly to on.)
First, on the dead circuit unplug everything (have you checked what "everything" is?) and turn all on/off switches off and turn only one switch in each 3-/4-way switch system of the circuit the other way. Try to reset. If the breaker stays on, one of those items you undid from the circuit has the short in it, so reconnect one at a time. ...[I don't feel comfortable plugging something in with a short inside it, because if I'm slow I could make some surprising sparks where I'm plugging it in. I plug a few in with the breaker off and then turn it on.]... That should narrow the culprit way down.
But if the breaker had retripped in spite of all the disconnections you made, then something more permanently live is shorting -- a receptacle, wire, automatic light, or hard-wired appliance, for instance. I would suspect outdoor things before indoor. In any case, pick a point along the circuit -- maybe midway along -- and disconnect the hots at that point. This is "divide and conquer": if the short is still there, it is electrically closer to the panel than that chosen point; if the short is gone, it is further out. By reconnecting what you undid and then opening new hots in the direction of the short circuit and by keeping track of all this, you should reach a place or a particular length of cable at which to look for the actual short. If it doesn't show itself, I or another electrician may be able to advise you more or find it for you. If you suspect a recent screw or nail is to blame, go to Screw.