Try power cycling the modem. To do so, unplug the modem, wait for at least 60 seconds, then plug the modem back in. After plugging it back in, wait for at least five minutes, which will give the modem time to be fully up and ready, and then see if everything is working as it should.
It's also possible that there is inclement weather at your gateway, though if this is the cause it should only be temporary.
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