Modem: Power cycling the modem or pulling the power and letting it sit for 30 seconds before plugging it back in, accomplishes several things. It resets the connection to the network and forces the connection to be re-established. It also causes the electronics and software in the modem to reload.
Personal Devices: Just as a modem has hardware and software that needs reset from time to time, your personal devices such as computers, tablets, smartphones, etc., all can benefit from being shut down completely and started back up, so the software loads fresh.
Verify cords are all securely connected
Phone: Pull the cord out of the modem and jack, then re-seat it back into the modem and wall jack to ensure you have a good connection. Swapping cable ends is a good practice.
Power: Verify that it is plugged in to a working outlet, has a tight connection both to the wall receptacle and to the modem Power jack.
Ethernet: Disconnect from the modem/router and device. Re-seat connections, this is a good time to swap ends of the cable.
Modem: needs to be unfiltered or using a filter splitter if the jack is shared with a telephone.