Once your ad expires, you still have the opportunity to renew it.
To renew a free ad, all you have to do it click on "Renew" as in the image.
If you wish to renew an ad that is posted in a paid category, you can click on the link "Renew" under the expiry date and proceed to the payment of the publication fee.
For your information, if the ad stays too long without being republished, it will disappear from your account. You can renew it within the next three months at most.