|Are you using the default logon password to access the router's configuration utility?|
If you are, someone nearby may be cracking your WPA key, logging on to the router config utility and turning off the wireless security.
Suggest you change the default router logon password to a unique one that only you know because the default one is the same for every router made by that manufacturer, so it's easy for anyone to find out what it is.
Then change your WPA key, or better still use WPA2 if available as it's harder to crack.
After making in any changes in your router config utility, don't forget to click "Apply" or "Save", otherwise your new settings will not be saved.