A few things to try:
Check your router is on/the lights are as they should be! If possible, get a friend with a wireless lappy to see if s/he can connect to your network.
Reboot your router.
Try connecting to your router by cable and check that all the settings are correct.
If a neighbour has set up a network using the same channel as you, it could be interfering with your set up - try swapping to a different channel.
Try resetting your router and rebuilding the network (there's usually a button you can press).
If a neighbour has set up a network using the same channel as you, it could be interfering with your set up - try swapping to a different channel.