Most modems come with a power button that enables you to quickly power them off. After a minute or so, turn your modem back on and wait for it to re-establish a connection. Once it does, check to see whether you’re able to access the internet from your browser.
Note that sometimes simply restarting the router isn’t enough. You may want to reboot it by unplugging it entirely, and then waiting at least 30 seconds before plugging it back in and powering it on again.