Your description is somewhat vague, but I'll try to help. Let's assume that the "box thing" is actually a modem/router. Steps to try:
1. Make sure that the client PC in question is actually communicating with the router. Enter 192.168.1.1 in your browser's address bar.
Do you get to the password entry for the config UI?
2. If that was successfull, open your client firewall. Look at its network permissions (the ones that you would have created - or it created for you - when the LAN was first established). Is your local LAN adapter listed? Does it have an IP range assigned to it? Is it described as "trusted" or similar?
I hope this information is not too technical for you to follow; networking is a technical subject unfortunately. Please post back promptly if you would like any further clarification/explanation.