NOT a system restore....

Start by resetting your mothers router, then restart the computer. Now, since you didnt tell me what operating system you are running or what wireless network card you are using or what router you are using... etc. I cant give you exact help but I will make some assumptions..

MY ASSUMPTIONS:
using a laptop
windows XP
typical namebrand card

go to Start>Settings>control Panel>Network Connections

right click your wireless network setup, choose view available wireless networks
click the advanced button, then click the refresh button, it should come up with a bunch of different wireless networks, in the bottom window remove the wireless networks that arent yours. then refresh it again, close the window then right click that card again and view available. reselect your wireless card.... it should work now