How can I fix an exterior electricity cable that I've managed to put a spade through?

Whilst working in the garden I put a spade through the electricity cable that runs from the house to the garage (the cable was buried in the bed alongside the garage wall). The cable is 2.5mm I think and I was wondering if anybody knew how I could go about repairing it? Would a junction box be ok, or would it need re-wiring due to it being in the soil? Also, if re-wiring is required does anyone have any idea how much this might cost?