1 - Do a full powercycle on your system. Unplug your modem for 25 seconds, router for 25 seconds.
2 - Also, the coaxial cable on the back of the modem, unscrew it, blow in the end, wipe down the tip and screw it back in.
3 - If that doesn't work, bypass your router and direct connect. Requires another powercycle on your modem after direct connecting. If this works, there's a problem with your router.
4 - If THAT doesn't work, check for a splitter in the wall with the coax cable and bypass the splitter and direct connect fully (Coax coming from the wall direct to your modem) If this works, there was a problem with the splitter.
5 - If that doesn't work, the modem is giving out and you need a new one, or the lines from within the house or from the pole to the house needs work done and you need a tech to go out there to fix it.