The outage started around 1:40 AM eastern time, according to several tweets. Comcast has not announced anything yet, and service technicians have only given vague answers to users who called about the problem. Comcast's own map only shows local outages, but Down Detector is showing this as a major, nationwide issue.
The service provider (a rival to Verizon, Engadget's parent company) experienced a very similar outage last June, and eventually chalked it up to cut fiber cables. Engadget has reached out to Comcast for more information about the problem, and we'll update the post once we hear back.
Update 8/7/2019 3:43 AM ET: Engadget reached out yesterday to Comcast but after an initial promise of more info, we haven't heard anything back.