no wifi connection (however network visible)

Today I can’t make a WiFi connection between my Ubuntu Lucid Lynx computer and my ADSL router. Though my network is visible in Ubuntu (network manager).

I can’t connect. Ubuntu does not stop trying to connect to my network, but fails. However sometimes it asks for a password. But using the right password does not help.

How can I solve this problem?

I also have a Windows machine connected to the router by means of ethernet. This works well.