I deal with this every day for the people I support. Here are the steps:

If the wireless icon by the clock says connected and it is connected to the correct network, right click the icon and select repair or diagnose and repair.

If that does not work, then unplug your modem and router for 30 seconds.

Plug the modem back in first and wait 30 seconds,

then plug the router back in and it should be up within a minute or so.

See if your laptop reconnects, if not try the diagnose and repair once more.

If that does not work, then you must right click the icon and go to "Network and Sharing Center"

Click Manage Wireless Networks and delete your current network.

Click the wireless icon again and it should say "Wireless networks are available"

Click that and reconnect to your network. You will have to re-enter your wireless key if you have one