If you haven't moved your wireless router and used to get a good signal upstairs it's possible that somebody who lives nearby has set up a wireless network and it's interfering with your own.
You can reduce interference by changing your wireless channel. This is done from the router's own configuration menu. Unfortunately it's not possible to instruct you how to do this without knowing the type of router that you have. For instance, is it D-Link, Netgear, LinkSys etc, and also what model number is it?