Could be someone parked in the street or living nearby who is hacking in to your router's configuration utility --- that's where your router's wireless function is usually enabled/disabled remotely.
If that's the case you've probably never changed your router's password for admin access, you've probably just left it at the default. You need to change that password to one that only you know, make it strong, a mixture of numbers & letters, uppercase & lowercase. Then write it down and hide it.