What factors can cause DSL to stop working?

The DSL modem is very sensitive to electrical spikes, and can freeze up if it experiences a strong enough spike or surge. Usually turning the modem off for 30 seconds, then starting it again and waiting about one minute is sufficient to restore service. The same thing can happen to routers, as well, and usually unplugging the power for 30 seconds, then waiting a minute or so after plugging it back in will restore service.
This one simple step can restore DSL service about 75% of the time.