ibook wifi login problem

Usually when I open my iBook it automatically detects my home network and connects to it. For the last couple of days it has only done so if I enter the network name (which isn't transmitted) and password manually.

In the past when it has done this, switching off Airport and switching on again has usually fixed the problem. When that didn't work a reboot usually did. But this time nothing I've tried seems to work. Anyone got any bright ideas?