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 firmware has fixed it). Upon reboot, disable the wireless again, and apply a strong password for GUI access. Then configure and enable the wifi again using a strong password.
IOW, the key to doing this correctly is keeping the router off wireless until you get that router updated and secured.