Sometimes simply turning the modem ON/OFF isn't enough. In those cases, a RESET can be helpful.
For step-by-step instructions, read "Modem RESET: Understanding what it does and when to use it." Be sure you read the article before resetting your modem. You don't want to accidentally erase network information you need, and doing a reset clears out any custom settings you have programmed into your device including: static IP entries, DNS, customized admin password, customized wireless settings, port forwarding, routing and DHCP settings.