no internet connection

So xfinity was in our neighborhood and they sent a notification saying there would be a service interuption and would be restored by 6:30am the next morning and that came and went then i called xfinity and they said it should be resolved soon and it never did so they sent a technician and he couldn't figure it out the only thing he could come up with was that it was an issue with our router/modem but it couldn't be because our router is brand new. So he got a wifi router from his truck and hooked it up and it worked. So i don't understand how. We have never had a problem with our modern before so all of the sudden after xfinity did something it didn't work anymore.