If the branches are still alive, then with luck, the leaves will regrow - but you must keep it well watered in dry spells, whether its still in its pot or in the ground now.

Hopefully the coldest weather has passed now, but that was the reason, a young tree, in a pot, and of a slightly tender type. As B has said, you should be lucky. About May time, you could prune it lightly to encourage it to branch out, and it'll be much better in the ground, in a sheltered spot, than in the pot.

Be careful not to give too much water either.
check if the direction of the dropped leaves position, if started from the tips (wilted first) the it's lack of water, if started the bottom upward (sometimes turn yellow first) then it's too much water. This plant should be sufficiently hardy where you live (Spain)

Thanks for all your help. Will persevere and see what happens.