Today when I came home, all LEDs on my Fritzbox were off. Short test with a voltmeter showed that the AC-adapter is ok. Looks like the router itself is broken. No router = no wlan and no internet. I'm disconnected from the worldinternet.

Fortunately, I had my old router stored somewhere in a box in my basement. Took about an hour to find it, to connect and to configure it. Now I'm online again.