What? Sounds like you got an over zealous phone drone.
Considering they were just trying to change up your internet settings, usually I find on trouble calls that they just screwed up the account settings. I don't know how Comcast specifically setup their account system but for Charter, each service has it's own individual package built into the system behind the scenes. And if someone screws up while creating it then you can't activate or swap equipment. Not that a power surge couldn't have physically damaged lines, it can happen but it's REALLY UNLIKELY, the tech will probably laugh if they know anything when reading the comments on that job.
Is the new modem actually starting up? If it'll actually get through, then I would suggest calling back to customer support, and asking them to double check to make sure the account is built correctly. However, since you did downgrade to just internet, they likely went out and put a physical filter on your line to block any tv signals, they could have put up a bad one and just didn't double check. It's also possible that the modem is already provisioned for somewhere else and will need to get swapped out or deprovisioned before you can actually do anything with it.
Another way you could check to see if there is something physically wrong (if it doesn't boot up) is to take the modem and connect it up to the infeed into your house. If you can get it going there then there may in fact be something wrong between there and where it was.