Some days I have to reboot my cable modem several times a day, as I am on a part of the cable company's network that supports the high school, and they reconfigure things bringing up additional equipment for that load in the morning, then shutting down as the load gets lighter. I suspect that the modem is still looking for a router no longer there, or has cached a routing table no longer valid.
Often a problem like this is in what the router has cached, or your PC has cached, but rebooting the router usually doesn't fix things for me, rebooting the modem does. Even so, it may be a few minutes to a hour before things get back in order.