I have had the exact same problem. Unplug your router & plug it back in. Mine kept doing that to the point I updated firmware, same problem. Finally just trashed the Cisco router and went to another brand. Not that Cisco is bad, but I've had the trusty old Linksys, newer Cisco and they did the same thing after awhile.
New router has been running about 2 weeks without any issues (so far)