The U.S./DS light blinks, but the online light does not illuminate.
That indicates that there is low/no signal or the modem may be broken. Did anything chage that might have caused the issue? About all you can doi is inspect your lines. If everything looks fine you'll need to call for service.
1-800-COMCAST (1-800-266-2278) or 1-800-XFINITY (1-800-934-6489)
Inspect the coaxial lines
Inspect as much of the coaxial cable line leading up to your cable modem for signs of extreme bends, cuts, loose or corroded connectors. If possible, try inspecting the coaxial lines from the point this cable enters your home up to the point where it connects to your cable modem. If you see a section of cable or any connectors that are damaged, try replacing the section of cable. Also, try disconnecting and re-connecting all of the coaxial connections that lead up to your cable modem to ensure each connection is tight, corrosion-free, and that the coaxial tip visible at each connection end is not bent. Coax should be RG6 with compression fittings.
Each splitter that is added to your cable line will degrade the cable signal slightly and may cause slower connection speeds or frequent modem disconnections. Normally, you should have one main splitter installed where the cable line enters your home with one branch feeding to the cable modem directly and the other branch feeding to all the TVs in the home. If you have multiple splitters on the coaxial line leading up to your cable modem, try by-passing these splitters and running a single coaxial line directly to your cable modem, if possible. This may disconnect any TVs that may be connected from these splitters but if this improves your modem connection, the problem can be attributed to either a faulty splitter that needs to be replaced or a potential signal problem that may require a technician visit. Splitters should be of good quality and broadband compliant. The label should read 5-1000Mhz (or 1Ghz).