Well, very likely the fault is in one of the joins, so you will have to start at the shortest length side and dig back to find the failed one, unless you did mark the cable run and have a diagram of the join location ( always a good thing to have, or at least a direct run so you have a keep out area for gardening and digging)to do only a short dig. Otherwise dig down at the ends for around 10ft and see if there is cable damage there, otherwise it is the join that has failed.