Try to connect your PC with ethernet cable as mentionned before, it helps, more than that try to replace the cables that are wired with your modem, a simple cable can cause such problems
You can also reset your modem, there is a small hole in your modem, use a pin and press it for 15 secs and it will be restarted. If not fixed you can try changing your IP adress throught PC settings, edit it from manually to editable and change last number of adress or last two ones. More than that you can contact your internet provider if not fixed and they will do their job checking the inernet/phone cables that are lined with your house from the street if there is a problem they will solve it(althrought if it wont be fixed they will tell you to check your house cables, phone ones that are installed in your house, rooms, etc. I got a solution for that if it's not fixed) try these steps before.