Usually your router assigns the computers on the network an IP, starting with 192.168.0.100, and then 192.168.0.101 for the next, and so on. If one disconnects and reconnects, it may not get the same IP the next time it connects.
In your router settings, you can sometimes set a 'DHCP reservation'. For example, my computer is set to ALWAYS connect as 192.168.0.136.
If your router doesn't do that itself, you can change your connection settings to do it. Instructions for that can be found
here.
After that, make sure the proper port is forwarded to that particular IP that you set for yourself.
Jam it back in, in the dark.