Reboot the receiver.
Tell us what model receivers (bad and good) we're dealing with.
Tell us which model dish you have.
Go to the test channels and tell us if any fail. If none do, go to the signal level menus and list the readings you're getting. You'll see readings for transponders on satellites 99, 101, 103, 110, 119.. though it's probably safe to skip writing down 110 and 119.
We'll also need to know about any other equipment upstream.. multiswitches and the like.
Has anything changed since the system was installed? Splitters, amplifiers.. anything else in the line?
While you're digesting all of that, follow the cable from the bad receiver back toward the dish, disconnecting, inspecting and reconnecting the cable as you go. It could be something as simple as a corroded connection or kinked cable.
Though your best option could be calling DirecTV and scheduling a truckroll. But do all of the above before you do that. Reboots and cable reconnections solve most issues such as yours. Assuming you haven't changed anything.