Check the Wires
It may seem obvious, but one of the easiest troubleshooting steps you can take yourself is to check all the cables and connections involved in your Internet connection. This is true no matter what kind of connection you have. Even if you're sure nothing has changed, it only takes a minute to make sure.
Start where your Internet service enters your house. This might be your cable company's line drop, a satellite dish antenna or a phone line. Make sure the cable is connected securely, and any cabling that runs outside the house hasn't been damaged by weather or chewed on by birds, bugs or squirrels. Just be careful if there are any electrical lines around -- and if you see damaged lines of any kind, don't touch them, just call the cable or phone company.
Next, follow the cables through your house, checking connections at every appropriate point. If you use a router, check those connections, too, and make sure the correct cable is going to the correct place. If your router feeds several different computers or gaming systems, it can get confusing to keep track of which cable goes where.
If the cabling and connections seem OK, the next step is to power cycle your modem.