Diagnostic Tree: Tripped GFI
Is There a Ground Fault?
If you have reached this page without beginning at the Start of the diagnostic tree, you may do better to start there.
A GFI device that continues to trip, immediately or later, is almost never defective itself (though it might have been misconnected recently; see GFIs), so you will be needing to look for a cause. You should acquaint yourself with what all has gone dead from this tripped GFCI because these things are where you will be looking for the cause. If the GFI is in a bathroom/garage/outdoor location, outlets in all those areas may be dead (but look around since you may not be aware of all of them). Likewise if the device is in the kitchen/dining/nook area, see which other outlets in that area (and maybe also outdoor outlets) are dead and which are not affected.
The ground-fault is an unintended continuity (a touching) of either the black wire or the white wire to the ground wire or to anything grounded (other than the neutral), like: grounded metal, the earth, or things touching them. Since this could come about within cords or appliances that are plugged into any of these dead outlets, unplug such things and see if the GFI will reset. If it will, plug things back in one at a time to see which is responsible. If it won't, note whether any of the receptacles is broken and whether they or the interiors of their boxes are wet and whether any (out in the yard) receive their power by a buried cable. Replace any broken ones, dry out the wet things and reset the GFI.
If it still retrips, undo the hots and neutrals of any buried cable at the box where it seems to leave the house, and reset the GFI. If it no longer trips, you need to reconnect and then repeat this disconnecting procedure at other boxes in the yard. If you have found nothing making contact from hot or neutral to ground or earth, then you will probably find the GFI is still tripping for one particular piece of buried cable. (So you will pull up, dig up and repair or replace that piece).
If the GFI tripped even when the line feeding out to the yard was undone, then open any dead indoor boxes (you may already have opened them) and look for hot OR NEUTRAL wires making contact with any ground wires, separate them, and reset.
If no contact with ground wires was found and you have been thorough in all the procedures recommended so far, I'm not sure what would be responsible for the tripping but you could write down the results of all the following procedures: Undo the hots and neutrals at a chosen point about midway among all the dead indoor suspects and attempt resetting. If it retrips, keep moving upstream (toward the GFI) among the dead ones and choosing another to undo; the cable, box, or receptacle to suspect will be whichever ones are left between the place you last undid and the one for which the GFI finally reset when you undid. If the GFI did not retrip that first time you picked an outlet to undo midway among the dead outlets, then reconnect it and repeat the disconnecting at another point downstream (away from the GFI); continue the reconnecting and disconnecting in this way till the GFI does retrip; this will mean that the suspect cable(s) or box(es) is from that point back (upstream) toward the previous outlet you undid.