If you are talking about the Home Hub 3000, the first light of three (leftmost one) is some sort of indicator that there is a message of some type from the modem.
Mine suddenly turned orange recently, and pressing it displayed the message "Your modem is working correctly" on the front panel display. (Helpful, eh?)
Despite the light, all seemed to be working normally though.
A reboot did not fix it. I had to pull the battery AND the power cable to cycle the modem.
After doing so, the light went back to normal