Typically your wi-fi access point (often built in to the router) will have 2 passwords. One is to access it - that's the one devices need to get on the internet. The other is a user name and password to actually administer the access point.
I think you probably put the first type on - someone was using your wireless access point to get on to the network. If you have the second you can change or look up the first.
Depending on the manufacturer of the access point the default administration password may be different. Sometimes it is admin/admin. If you share the make and model of your access point or router we can help you look up the defaults. We can also look up its default IP address for access.
Edited by MiCarl, December 25, 2015 - 05:12 PM.