iPhone 4 not see my home WiFi network anymore

All of a sudden, a few days ago, my wife's iPhone 4 (initially with iOS 6.0.1, now with iOS 6.1) does not see anymore my home WiFi network (created by a Time Capsule), to which it connected automatically.

This has worked flawlessly for years but now the home network (nothing has been changed to its configuration) does not appear in the list of networks seen by the iPhone 4 (mine, iPhone 5 - iOS 6.1 - has no problem at all) and when trying a manual connection I get an error message saying that the network was not found. When I tested, I was a feet away from the Time Capsule.

I have turned the iPhone off and on again twice, to no avail. Just to try something, I created a new WiFi network from my iMac and it was immediately seen by the iPhone 4.

I don't see what could explain these symptoms and what would be the cure.

Help welcome

Thanks

Jacky