The signal being transmitted by your router has limit to how far it can travel. It is normal for a wifi signal not to cover both floors of a two story house. You will need to invest in an Access Point or Wireless Repeater/Extender. If you have the option to lay a cble to the ground floor you can use an Access Point to your router. If a cable is not an option then a Wireless Extender or Repeater can connect wirelessly to your upstairs router and extend the signal to the bottom floor. Either one of these options are very expensive or hard to set up, with less than 50$ you will be set up the ground floor and connect to your network with a strong signal.