Your ISP (sometimes the Configuration reset at the Server Level) is responsible or probably your Modem have gone faulty!
You need to bring it to 'Factory Default' & configure again..
Here's How::At the rear side of the Modem/Router, you'll see a Hole or a Button to reset it for Factory defaults, use a Pin or sharp object to press it persistently till all Lights of the Modem goes OFF!
Open
http://192.168.1.1 in the address bar of the Browser.Punch in 'admin' at both 'Username' & 'Password' fields. Run the 'Smart Configuration Wizard' to answer easy questions....& its done!
Revert for more inputs as per your observation....