Sometimes if you can't get online, this is a good first step in trying to fix things. If it saves you an hour on the phone with tech support, it must be a good thing, huh?
I've found with our modem (DSL, not cable, but same basic idea), it only takes about 30 seconds, but I almost always have to reboot, so I just shut down the computer before I start the recycle process with the modem, then start it back up once the lights are back on the modem.