Have you port-forwarded?
If not, follow these instructions and port-forward your computer so you can host a game.
Click here and then select your router make then model.
The reason you might not be able to host now is because you used to host via
UPnP, which in my experience has been untrustworthy, randomly not finding my router.
Could also be that you have a dynamic IP, so to your router your identity might be 192.168.1.3 one day, and the next 192.168.1.12.
Causing the ports to not be forwarded to your computer.
If so, follow this instruction
here to set up a static ip.
Hope this helps.
