What should i do if i won a giveaway but the key cannot be activated in my country?

Inform the creator of the giveaway, just in case he has another key.

If he hasn't, I always allow him to delete the giveaway.

From the FAQ :)

What happens if I create a giveaway and the winner is unable to activate the gift in their region?

If you setup the giveaway with the appropriate region restrictions, and the winner was outside of those regions due to inaccurate information they provided, then you can request a new winner for the giveaway. If the giveaway did not have the correct region restrictions assigned to it, then one of three options are available.

You can acquire a replacement gift or key for the same game, which the winner is able to activate in their region.
You can accept the giveaway as being "Not Received" by the winner.
You can submit a ticket to have the giveaway deleted. In order for us to delete the giveaway, the winner will need to post a comment on the giveaway saying they agree to have it removed.

The nicest thing would be to let them delete and remake it as there are so many nonsensical region restrictions on things it can be very confusing for giveaway creators!