1. Try turning the router off and leave it for at least 20 seconds before turning it back on again
2. If 1 fails right click the connection under 'Network and Sharing Centre' and then click properties. On the window that pops up choose diagnose. Windows should identify the problem and fix it automatically
3. If 1 & 2 fail go to 'Network and Sharing Centre' again and click 'Manage Wireless Connections' on the left. Find the connection you're having problems with and remove it. You will then need to re-enter the credentials to join the network again.