I assume you plain can't see the router again so don't even attempt to connect to it. Assuming that's true :
1. Reboot the router, see if it clears issue.
2. Take your laptop and stand right next to the router. See if you can see it. It's not beyond he realms of possibility that interference is causing the issue (DECT phones, video senders, etc, all share the same airwaves) and standing next to the router is a way of reducing the chance of interference.
3. No joy, then plug in cable and make sure you can log in to the router. Check the settings.
I've had a dodgy router in the past that has managed to turn off the wireless all by itself. Also the same dodgy router had a habit of dying totally - you couldn't get in to configure it and needed to reload the firmware onto it.
4. Try a different wireless channel. You will need to go into the router and configure the 'wireless setting'. There should be an option on there, so if it is set to (e.g.) channel 1, then chose channel 12. Etc. It may take a few seconds for the laptop to recognise the wireless after you have changed channel.
5. A couple of applications I have found useful as
XP: Netstumbler (
http://www.netstumbler.com/ - site seems down)
Vista: Vistumbler (
http://www.vistumbler.net/)
These sniff out any networks your PC can see. Helpful to see if it recognises your network, and also to avoid channel clashes with neighbours if you try step 4. They are relatively easy to configure/use -hit the play button(for netstumbler you may need to change 'device from prism to ndis depending on your network card).
6. The only other thing I can think of is disable wireless security temporarily and see if you can suddenly see the router. Last result and you want to re-enable it ASAP so make sure you know your settings, etc just in case the router doesn't retain them between disable/re-enable.
Wireless can be a royal pain in the arse when it doesn't work!
