Contact them at the phone number on your bill or 1-800-Comcast, or chat with them at https://www.xfinity.com/xfinityassistant/. If they can't fix the problem remotely insist that they send a tech out to identify the cause and correct it.
If the tech finds bad coax, splitters, amplifiers, or connections in your home (even if Comcast originally supplied them) you'll probably have to pay for the visit unless you have their Service Protection Plan (https://www.xfinity.com/support/articles/service-protection-plan, closed to customers that don't already have it). If the trouble is due to a faulty Comcast phone modem or anything outside your home, you shouldn't be charged.
Comcast may or may not respond to your message here. If they do respond, it may take them several days.