Tell us the name of the Modem, and the Router.
oh and
Turn off, disconnect, or unplug all the devices connected to your router (basically turn off anything that could be using throughput.), then power-reset the modem. Once the modem fully lights and boots up, reboot the router. <--(this process can take up to five minutes or as little as 30 seconds.) Now take a device other than your computer (aka an iphone or droid or moms laptop, (preferably a phone)) and run a speedtest.net test. There is also a speedtest.net app available.
Write down the exact results.
If at that point you are still experiencing a problem, call Comcast and say your service is out (again.)
Keep in mind that sometimes cable companies throttle down the internet while fixing fiber optic cables and such. This never lasts more than a week.
I don't have time to be any more detailed or ask you good questions but good luck.