An unsecured WiFi?
Lately I've seen a couple and heard about some hacked modems even in what you'd consider very safe neighborhoods where the risk of an unsolicited user might change the settings.
If you can, reset the router to factory defaults. Change the administrator password, set the WiFi security to WPA2 with a strong password. Disable WPS i.e. the push button password negotiating system. If possible, restrict the use for MAC addresses of your known devices only. Then set the DNS to 8.8.8.8 (or one of them if there's several options). As it's Google's DNS server it's well maintained so it mostly works. If you have a separate modem, do the same for it.
In case it's a more public WiFi, contact the administrator and ask him to do the above.