Wifi or Ethernet?
Check to see if Comcast is reporting an outage for your area (see https://www.xfinity.com/support/cable-tv/outages-in-your-area/).
If no outage is reported and you want to troubleshoot this yourself, please see Connection Troubleshooting Tips. If you still need help, please provide Information Requested for Connection-Related Posts.
If you are using Wifi it's best to switch to an Ethernet cable connection if possible while measuring speed and checking the reliability of your connection to Comcast.
If you can't find the problem or you'd rather have Comcast do the troubleshooting, call them at the phone number on your bill or 1-800-Comcast, or chat with them using the Internet Support option at https://support.xfinity.com/chat. If they can't fix the problem remotely, insist 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 or sign up for their Service Protection Plan (https://www.xfinity.com/support/articles/service-protection-plan, about $6/mo) and keep it for at least 60 days. If the trouble is due to a faulty Comcast modem, eMTA, gateway device, or anything outside your home, you shouldn't be charged.