|I would first recycle the power on the modem (unplug, count to ten slowly, plug back in) and see if that doesn't fix the issue.|
If the problem persists, you should contact your ISP. Typically the modem is your provider's property. There are however, ISP's that "sell" it to you and it's your problem. Regardless, most ISP's have the ability to remotely log into the modem and run diagnostics on it.
If recycling the power on yours doesn't fix it, call your ISP and have them take a look.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.