1) Do you connect via SACM (stand-alone cable modem) or STB (set-top box)?
2) Try:-
Power everything off
Power on modem (or STB)
Wait for lights to settle
Power on router
Wait for lights to settle
Power on PC
Should be job done. If you are on an STB, you will either need to clone your MAC address (a router option), or register the router with NTL.