Ok since you already port forwarded you know the steps, here are some solutions:
1. Since you have a new modem your router IP and your Computer IP may have changed.
2. Check your network settings and find your computer's Internal IP
3. I'm not sure but your external IP might of change not allowing people onto your server.
4. If you have a new modem your port forwarding program might be different.
5. We need more information: What modem, Will your server even bind the port, What's the problem with the forwarding