Unless you simply failed to use a wifi password, or maybe it was too easy to crack, then perhaps you've become a victim of the KRACK vulnerability.
All passwords should at least be 14 chars in length, and contain letters (both cases), numbers, and special characters. If you visit http://grc.com/pass, it will help generate good passwords for you.
But first things first.
Reset your router to factory defaults and immediately disable the wireless. Use an ethernet connection instead. Then apply a strong password for GUI access. Visit your router manufacturer's website to get the most recent firmware update and apply it (I'm assuming your current firmware is old and pre-dates the KRACK vulnerability, and hopefully the most recent...