If your auction had bidders - once its ended you'll be able to see the status of the auction.
You can directly message the buyer to discuss the item - or give them a gentle reminder they only have 3 days to submit their payment.
But if they don't make a payment in time...
If the buyer doesn't send the payment within 3 days - you can mark the auction as not paid. This will give a blackmark to the buyer - and prohibit them from bidding on any other items for 3 days. You should only do this if they have failed to reply to any messages and did not send any payment.
If your buyer does not submit payment - don't worry - you can just list your item for sale again - or message the second-highest bidder and ask them if they are interested - then send them the link to the new listing page.