Hi Pompom,
Sorry to hear of your misfortune.
You need to establish the date and possibly time exactly when the car was destroyed - a written note from the Guardia Civil asap is essential.
If the date was before renewal date you are entitled to a 100% return of the premium paid in advance for the new year. If the date the car was destroyed (not stolen) was after the renewal date you are not entitled to any refund.
The above is based on two established "rules" in insurance surrounding the principle of indemnity. Firstly, you cannot insure anything that does not exist. Secondly in a write-off situation the insurers have fully indemnified you in paying out and therefore there is no refund for what would be a mid term cancellation of the policy.
If the Guardia Civil do not know the exact date try to get them to give you the likely earliest date it may have occurred and the date found, again in writing.
If you have any queries feel free to send me a PM if you prefer.