Since yesterday the wifi stopped working

Hey ​@Lukas_z 

That sounds irritating at least. There is a wifi button on your modem, if the light isn't on you can hold the button for about 2 seconds and it should go on. 

If your modem keeps restarting itself there is a fair chance it might be a defect. Try to leave it unplugged for about 5 minutes or give it a factory reset by pressing the reset button in the moments it is online. If it doesn't work, you can schedule a call with KPN to check and resolve your problem by clicking here. I added a screenshot of the button to press. 

Nothing has changed, the process is the same. Red power button, then the internet, power, and TV icons flash (no Wi-Fi), and then the same thing happens again.

Schedule a call with KPN using the link I posted above. Modem seems to be broken, they can send you a new one after they run some tests remotely! 

Good luck, sorry I couldn't be of more help!

Hi ​@Lukas_z, welcome to the community. I agree with Flitsend, this sounds like something that is best checked via the phone with a colleague there. The quickest way to do so is make an appointment like he mentioned.

If you've already done that it might be fixed already now.

That's what I did. The helpline employee suggested it might be the cable. I just connected a new power cable, and the modem no longer reboots constantly, but the Wi-Fi doesn't work. What should I do now?

Wi-Fi started working. The cause was the cable. Thank you very much for your help

I'm glad it was fixed that way ​@Lukas_z! I think you have a Box 12 modem, is that correct? There is a specific range of that modem that benefits from replacing the power adapter. It looks like yours is one of those, and it was solved that way.