You do not say which operating system is being used. I am assuming since you moved the router and PC downstairs that you are talking about a wired connection? However, in both instances
First thing to do is to check the LAN settings.
If you are using windows 7 Click Start, and then click Control Panel. Click Network and Internet, and then click Internet Options. In the Internet Options dialog box, click the Connections tab. Click the LAN Settings button - there should be a tick in the "automatically detect settings".
Then you should look at TCP/IP settings - again in windows 7 Start-Control Panel - in search box top right type in "adapter" In Network sharing centre - choose view network connections Right click the network - should be Local Area Connection - click on Properties Go onto the networking tab and scroll down to IPv4 then click Properties Click "obtain an IP address automatically" - click ok