If you paid the purchase, but did not mark the payment complete before the payment time window expired, please contact the seller. The bitcoin seller has functionality to Reopen the purchase. You can contact the seller and your other existing trade contacts under Dashboard in your LocalBitcoins.com user profile. Send a message to the seller and kindly explain your situation.
Reopened purchases get escrow protection re-enabled until the seller can confirm they payment has successfully arrived.
If the seller does not respond to this request for 48 hours please contact LocalBitcoins.com support using Support request form and mention your trade contact ID number.