If you encounter a scenario where your transaction is stuck in "pending", try the following:
1) Reset your metamask by going to settings -> Advanced -> reset. Then refresh page.
If that does not resolve the issue:
2) Repeat method 1
Empty browser cache and delete cookies for Zero Exchange. Then refresh page.
If this doesn't work, you've probably used the incorrect gas settings and your transaction is being processed on Ethereum chain. Confirm this by checking Etherscan:
You can cancel your pending transaction by using this website: https://cancel-ethereum-transactions.web.app/ and entering the exact same nonce number.