Your money is still with us. It does not reach the seller until the item reaches you, so nothing is lost while this is sorted out.
In order
1. Check the tracking. A long quiet gap is common; a stalled parcel is different. What the statuses mean.
2. Give the stated window. Festival weeks and remote PIN codes add days. The estimate came from the seller and the method.
3. Message the seller on the order. Most of these end here — they know where it is.
4. Raise an issue. How. A person reads it and answers within 48 hours.
If the courier lost it
It happens. The claim is between the seller and their courier; your position is simpler, because the money has not left. You are not chasing a refund from someone who already spent it.
Questions people ask
How long should I wait before raising it?
Past the estimate, with no tracking movement for several days. Earlier if the tracking says delivered and it is not.
Will I get the delivery charge back too?
If the item never arrives, you are not paying to have it not arrive.
What if the seller stops replying?
Raise it. That is what the route is for.
More: how buyer protection works.