Why is our internet so incomprehensibly slow when it didn't used to be?

I was hoping I wouldn’t have to call Comcast again since the last time I did there wasn’t even a problem and the guy treated me like a total moron. But our internet was over 15 Mbps according to Speedtest.net a couple weeks ago, now we are down to 0.7 Mbps and it’s been like this for a few days. What would cause such a dramatic change in internet speed? How can I tell if it’s a problem with the modem or the router or the connection itself?