Some questions come to mind:
Do you have a static IP address from your ISP?
Dynamic DNS?
Have you verified the listening service is a box you own?
Is there a reverse proxy set up?
Checked the edge router logs to see if it rebooted recently and reloaded firewall rules?
What else sits between your router and the listening server?
This could be any number of things, maybe this will help point you in the right direction.
No joke, I thought for a second you were referring to Infinite Solutions and were trolling hardcore. I haven’t thought about that in a long time.