Short answer. Since you cannot return the thing itself because you lost it through your own fault, you must return the fruits you received and the value the thing had at the time of the loss, plus interest on that value running from the date of the loss.
What the law says
Whenever the person obliged by the decree of annulment to return the thing can not do so because it has been lost through his fault, he shall return the fruits received and the value of the thing at the time of the loss, with interest from the same date.
Civil Code, Article 1400 — Loss of the Thing to Be Returned. Read the full provision →
What replaces the thing itself
Where annulment obliges you to return a thing you received, and you cannot do so because you lost it through your own fault, the law substitutes a specific package in place of the thing: the fruits you received from it, and the value of the thing at the time of the loss. Since you can no longer hand back the item itself, the statute converts your restitution obligation into these monetary and in-kind equivalents.
Value is fixed at the time of the loss, not today
The statute is specific about the timing: the value owed is the thing's value at the time of the loss, not its value when the contract was made, nor its current value if that would be different. This fixes the valuation point precisely, which matters if the item's worth changed significantly between when you received it and when it was lost.
Interest runs from the date of the loss
On top of the thing's value, you also owe interest, and the statute specifies this interest runs from the same date — the date of the loss itself. This means the interest calculation is not tied to the date of the annulment decree or to when the case was filed, but specifically to when the loss actually occurred, compensating the other party for being deprived of that value from that point forward.
This rule presumes fault on your part
This particular consequence is triggered because the loss happened through your fault. The article does not address what happens if the loss occurred without your fault — that is a different scenario governed by separate rules on restitution after annulment. The fact that fault is what activates this specific obligation to pay value and interest, rather than simply excusing you from returning something you no longer have, reflects that annulment restitution still holds a party accountable for losses it caused through its own negligence or wrongdoing, rather than letting fault-based loss simply cancel the restitution obligation altogether.