Do you have another source, like a DVD player connected, and if so, does it also have no sound?
Also, how do you get TV, is it an HD cable box, possibly connected to the set with an HDMI cable?
Wonderful. Here's what you have to do.
Just unplug the FIOS box from the power for 30 seconds, plug it back in, and that should fix it.
But more than likely, this problem will happen from time to time, and the permanent fix would be to not use the HDMI cable, just use the 5 wire component video connection. Make sure the FIOS box knows to deliver HD via this connection (it probably already does).
HDMI holds great promise to make things interconnect and communicate with each other more easily, but it's fussy, and it's prone to developing oddball problems like this.
Hope that helps, let me know if there's anything else I can do for you, and thanks for using JustAnswer!
I'm delighted to help, and thanks for the Accept, I appreciate it!