Assuming the workflow is still active and the server didn’t reboot or anything like that.
Do you have a log of the sending Application stating it does actually send the Webhook?
Are you using Desktop or something else with the tunnel option? The tunnel URL can change if the app crashes or the network connection drops which is why we don’t recommend it for production use.
I guess that answers the question of how you have n8n setup
So if you are using desktop it is a bit tricky as you will need your own domain and to set up port forwarding on your router so that the outside world can access n8n.
Another option could be to use something like ngrok and use that as a tunnel solutions. To change the URL you can find the ngrok steps here: N8n Desktop custom Webhook url ngrok - #4 by Jon the same theory would apply to your own domain as well.