First, power off your modem/router for 5 minutes, then start it again, sometimes that works.
Or If you're on DSL, check your phones, phone cables, your splitter or anything else connected to your phone lines. I had this a couple of days ago, simply disconnect everything from your phone lines, like wallplug, the phones themselves and everything, then plug just your modem into the wall with the shortest cable possible, see if your speed increases. If it does, add back phones and cables one thing at a time until it slows down again, then you know what your problem is.
If you're not on DSL, or that doesnt work, call your ISP