If your signal strength is truly a "flat zero" it means your LNB and your receiver are not communicating. If it was a case of the dish not being pointed correctly, or if the dish had been moved slightly, or it were a line of sight problem, your signal strength meter on your receiver would show a signal of, say, 20 or so as a result of ambient "noise" that exists in the air.
A sudden drop of signal to zero indicates that something catastrophic has happened. The first thing I would suggest that you check is your connection between the receiver and the dish. Coax connectors can become corroded, even though they may appear to be OK from the outside. It's also possible that your coax cable has become cracked somewhere along the run, thus shorting or breaking the connection.
The other possibilities are that the LNB or the receiver has become defective. The only way I know of to check this is to either substitute another LNB or receiver as a test, or purchase an external satellite meter and insert it in your coax to see whether you are receiving a signal (1) at your dish and (2) at your receiver. Also, be sure to try connecting to one of the other ports on the LNB; I've heard that one port can be bad, yet the other three will work.
